User blog comment:LDR/Possible Reaper Motive/@comment-1962336-20120224024939

This is what I've been assuming since Mass Effect 1 (warning lots of science!):

We currently believe the universe to be open, meaning it has no boundaries. Matter is finite, and if it is true that the universe is infinitely expanding, matter will spread out too much to form new stars and thus new life (albeit trillions upon trillions of years from now). The Reapers are not truly machine, they refer to themselves as a combination of organic and inorganic material. They harvest races once they reach deep space travel, use the genetic material to create another reaper, catalog the race's (knowledge, history, etc.), and then leave no trace of the genocide.

Perhaps this is a way of recording life in our galaxy before life can no longer be created somewhere down the road. We do not know if the Reapers are from this galaxy, or how old they truly are. I really hope ME3 gives us a long awaited "why", otherwise it will be science fiction's greatest mystery IMO.