User blog comment:LDR/Possible Reaper Motive/@comment-1267137-20120225034903

I saw something on the bioware forums that basically basically said the reapers show up every 50,000 years and extinct the galactic civilization with the intent of having the most worthy species become reapers while the inferior species are either eliminated entirely or are re-purposed.

that's not the full explanation and the person who originally said it would certainly have said it better but that's a brief summary