I think the main reason is to customize your gun to fit you or the way you play . The old days it was to run bigger bottles without the need for gorilla arms and get a better balance .
Someone said it raises your hopper profile but compared to the old RH feed with elbows and revolution loaders today's center feed, low profile feed necks I don't see it making to much difference .
For me it brings the gun to eye line more naturally and therefore quicker