>>Very simply, it is a perk for frequent flyers. Delta boards in order of 1) anyone needing special assistance (which should be wheelchairs & baby carriages but is abused at times), 2) first class and diamond medallion, 3) skymember premium plus those who've paid extra for a "comfort" class seat, then, zones 1, 2, etc.
It's one of the few free things they can offer frequent flyers (as opposed to a free upgrade to first which they can instead sell to a coach pax for dollars or points).
Logistically it is a little backwards since the groups who board first tend to be toward the front and on the aisles but getting on early gives you a much better shot at the much-coveted overhead bin space.<<
Thanks for the interesting answer. But...but...it is a LOT backwards and it is absolutely the reason for congested aisles.