1. When you're traveling to the third world
Let's be honest. We all wish we had the mad traveling skills of Indiana Jones and could make it through any traveling situation seemingly unscathed, but there's a difference between Hollywood and reality.
While these can be some of the most impressive countries that will leave you dreaming of going back to all their uniqueness, they are considered third world for a reason. It's difficult to find the best places to stay, great places to eat and learn all the customs that you need to know to stay safe in any country. But when you're going somewhere where Internet advertising isn't a marketing must, these things can be especially difficult to figure out.
Sometimes it's just easier to have a local or someone else that's been their frequently figure it out for you. Especially since chances are that they'll be able to take you to some places you wouldn't have even thought of.
2. When you don't speak the language
Personally, I like the challenge of trying to get around and survive in a country where I don't know what they're saying. It means taking twice as long to get somewhere, not getting what I thought I ordered, or getting seriously lost, but that doesn't really bother me. For some people though, even the thought of not being able to speak with someone is stressful. If that's you, then take the tour!
Seriously, they make the whole thing so much calmer. most Americans assume that if they're in a first world country that there will be someone there that knows English and sometimes that is the case. The issue is that that person that speaks English may not be in the mood to be your personal translator. And especially if you leave the major cities, the chances of finding an English speaker becomes more rare.
If you sign up for a tour though then they'll do all the communicating for you. Which also means you don't really interact with any local; this can be a positive or negative, depending on who you run into!
3. When transportation isn't easy
A lot of countries are pretty easy to get around between taxis, rental bikes, public transit, etc, but if you're going somewhere that's not as accessible, a tour guide might be able to help with planning your travels.
I don't think this is a big deal if you're planning on just staying put in one city or area but if you're planning on going all over a more rural country this can be tricky. You can't just take a four hour taxi ride and there may not be cars to rent. I've had great experiences with locals when traveling but something else to consider is that travel can be safer when you're with someone that the locals aren't willing to take advantage of.
4. If you're a stressed traveler
Travel can be stressful even if you're someone that's accustomed to traveling all the time. Every person, country and situation are different so you never know what you're going to get or how you're going to react to something new.
I'm pretty comfortable traveling but I remember one time when trying to get from Venice to Florence I booked a train ticket and didn't realize that I was misreading the military time and was totally sitting right there as my train left. Then I had to get a new ticket which was super stressful since the machines were only in Italian. By the time I finally found a cafe with wifi to look up places to stay once I got into Florence, I realized that the hotel wouldn't be open by the time I got there, there wasn't a hotel available in Venice, and my new train ticket was going to leave at 3:30 am . It was a stressful mess trying to figure everything out in Italian (which I don't speak).
The big benefit was that I was in Europe (which obviously still has its safety issues like all countries) which made me feel a little safer than had I been traveling in a country like Morocco at the time. Now someone that spoke French or Arabic may have felt more comfortable in Morocco. So know you're comfort zone and know how far you can go out of it before feeling freaked out.
A tour plans everything out so you don't have to stress about figuring out the tiny stuff. This is also great if you want a quick getaway and don't have time to plan it. Don't let the fear of being stressed keep you from seeing somewhere amazing. Normally the places that are the farthest from our comfort zone are actually the most impressionable.
5. When you're worried about safety
We all have different comfort zones. If you're traveling somewhere where you don't feel comfortable it's going to be harder for you to really enjoy your experience there. There's something to be said about safety in numbers. Just keep in mind that the more tourists you surround yourself with, the less locals you're probably going to have the chance to interact with.
That may be something that's important to you or it may not be, just remember that in the end, this is your trip; you have some control over the experience you get.
6. When you're concerned about money
Now normally I think that going it on your own turns out significantly cheaper but if you're heading to a place where you can't even guesstimate the cost of traveling for a day, a group may be something to consider. That way at least you have a set price that already factors in sight seeing, food, hotels, etc. A lot of these groups get discounts too for bringing such a large amount of people so sometimes you can stretch your money further.
7. If you're traveling with a large group
Yes, they may all be your best friends. And yes, maybe you never fight. But travelling, especially in a foreign place can really bring out new sides of people that you may have never seen before. If you're with a travel guide they can act as a mediator. Instead of everyone trying to agree on where to eat, the tour already has a place. Instead of everyone trying to figure out how much time to spend at the Colosseum or what time to get up, the tour has a schedule for you. Less arguments, less stress, more fun.
8. When there's an option for a "fun" tour guide
Okay, I'm young so a "fun" tour guide might not be important to everyone but what I mean by that is that there are all different types of tours. I recommend finding one that's similar to your age group and extensively looking into reviews. If you go on a trip with college kids you're going to have a much different experience than if you go with a group of architects. It all depends on what you want to see and the people you want to be surrounded by.
Regardless of whether you choose to go with a tour or prefer to strike out on an adventure, the important thing is that you're getting the experience that you feel most comfortable with and enjoy your trip.