What can folders in Lightroom 5 do that collections or smart collections can't do, or can't do in a convenient way?
At first glance, it looks like folders are just a more limited version of collections, with the additional restriction that a photo must belong to exactly one folder at any time, no more, no less. What do I get in return for accepting this restriction? Why does LR have collections and (if I may call them that) a crippled version of collections? I'm assuming there's a reason, and I'd like to know what that is.
Note that I don't have tens or hundreds of thousands of photos . Are folders useful only for such people?
 And if I had, I could use collection sets, which can contain collections and further contain other collection sets, giving me all the power of folders.