Its there to get money off the new console without selling the old games
i bought the PS2, i sold my PS1 for $25 but kept the games that i could still play using the PS2.
i'll do the same with my PS2 if PS3 plays PS2 and PS1. Why take up space with old equiptment when the new stuff does everything the old could do. and as Darren pointed out, it becomes less of an issue each generation because theres not a huge difference in graphics.