User blog comment:The Milkman/Reaper Logic/@comment-3226495-20120430182906

I came up with a better reason for the reaper's existance then what the Catalyst gave.

Think of a large forest -- the galaxy -- and as trees grow -- species -- they take neutrients from the soil, if too many trees grows too closely, they have to share neutrients so they can still exist, but they can't grow anymore. Lumberjacks -- Reapers -- clear forests of trees to allow the remaining trees to grow more.

For insance, the Protheans were imperialists, if the reapers had never wiped them out then the humans, turians, asari etc, would all be slave races of the Prothean Empire. Since the protheans were dead we had room to grow as we wanted instead of being dictated by them.