Sure you can go at it alone, but for some people, guided tours offer the best of both worlds: a chance to travel and not having to stress out about all the details.
Here are some benefits:
1) You get to learn about the culture of a place
Yes, you can see all the great spots, but a tour guide really can explain to you insights that you are unlikely to find just by going at it alone.
2) You get to socialize with like minded people
Anyone who has taken a cruise and dined with the same people knows how you can build friendships that last a lifetime. Imagine going on a tour, then!
3) You don't have to stress
Isn't your job stressful enough? Why do you want to stress about vacation?