I think for people who do group tours;
-language problems are lessened
-no thinking about the vacation schedule (someone else plans it, except for the free time, which on a Japanese tour there is usually very little of)
-let someone take care of you
-never be lonely
Last time I went to Korea it cost over 50,000 just for the ticket alone. Had I gone at the exact same time with a tour, it would have been significantly cheaper, and the hotel charge would have been included. I know because I checked.
Depends on the tour and the length of time you're planning to go. We talked with our friend who went to Australia. She did see a few more places than we did (but sometimes 2 hours in one place), but she spent significantly more for one person than we spent for two (and we were there one more week!).
I'll give you another example. We generally go to Europe for 2-3 weeks every year. Most of the Japanese travel agency tours are for 5-7 days! When I say 2-3 weeks, I mean the actual time in Europe where as the Japanese tours are including the flight time as well! To top it off, we spend far less when you average the costs out per day (usually less than the package price as that would be per person, not for the price of two). Now of course, if you can only take off a week, then I suppose the tour might fit your situation, but it a lot more expensive
We do some package tours in Japan, which can be cheaper, but what might be booked as a group tour (for the hotel bookings and sometimes the train as well), you basically travel on your own. In other words, beyond staying at the same hotel and riding on the trains the same days, you plan the rest of the schedule yourself.