The idea is that one uses one's GPS (there are apparently people who do this with compasses and topo maps and sextants and so forth, but I for one am really not that dedicated to anything, and I do write my books mostly on computers) to get into the general location of a cache, and then one resorts to observational skills and a fine-honed knowledge of human psychology to figure out where somebody might have hidden a plastic container full of toys. In practice, this means that one blunders through mosquito-infested swamps until noticing the glint of plastic. *g* (There are refinements, but this is the basic idea.)
When [IF!] one finds the cache in question, one makes a note in its logbook, and swaps one of the objects within for something one has carried in.
...and the reason we do this, as my mother once said to me when I was quite young and she was explaining why people might drink to excess, is under the supposition that it is fun.
(Guess where I get my sense of humor from.)
I am now richer by one plastic shark, a deck of playing cards, and a resin dragon with a repaired wing. And boy does my lower back hurt.