When bookshelf speakers are placed on stands instead of a bookshelf or table there is more room for adjustment. The stands can be manipulated and placed away from the wall and the soundstage, depth etc. can be adjusted more accurately to one's liking, but the same can also be said for tower speakers. There is some truth in that the stand can help isolate the speaker from the floor (good for apartments and improved sound). Some stands resonate and can make things worse. Some put sand or lead inside the speaker stands. It's really difficult generalize and say that bookshelves are better than towers or vice versa. All things being equal, towers generally extend the low end more when compared to a bookshelf. There are too many variables involved.
The bookshelf speaker "thing" is fairly recent. They were not as common when I started this hobby in the 80’s as they are today. Budget, location, room size, neighbors or no neighbors, apartment or house, space, aesthetics, convenience, "the wife"/ (or husband) listening styles, music genres, playback volume, occasion and many other resons I’m sure.
Personally, I have a couple of each type in 6 different rooms. I like the bookshelves with 2 subs for background music and towers or the PA speakers for when I want to rock and get my fix!
Obviously, my comment is brief and only scratches the surface. If you can do it, why not have every combination and get the full experience? Life's short!