Of course there's no real value so many would find it useless, but because of trophies i play my $60 game more than once. It creates replay value and allows me to play the game differently, exploring more of the fine work the devs put into the game.
Before i'd sometimes rush through the game just to finish it once and i'd probably miss out on some of the nice work devs put in the game.
When i hold up a game that i've gotten all the trophies for, i can look at it and smile knowing that i've done practically everything this game had to offer me.
As well with trophies you can level up yourself =)