There is a lot of great opportunities to check out some places far away.
When you travel by yourself it can be pretty lonely but if you travel in a group you may fare way better !
National Geographic, REI, Abercrombie & Kent and many more will take you by sea, train, trail, bike, or hike to some of the great destinations.
Is a guided travel affordable ? Yes,
Is guided travel worthwhile? Yes in small groups you will get the opportunity to meet others and have sincere attention from the tour guide.
Do I need a guided travel when I am going to a place I have been before? I think that is the best reason to go on a guided travel because while you may have been to Alaska maybe you missed something, you traveled to Paris but didn't see the culture, or even Yosemite you didn't get to experience a night camping under the stars properly.
Why taking a guided tour is so expensive ? Remember when you spent day in and day out at the office on spreadsheets and presentations you were working and the value of your work was presented in your paycheck, now you have someone working for you guiding you thru a terrain that you are not familiar with, providing you meals, and ensuring your safety that is enough for the cost to be justified plus the guided tours will answer your questions.