Basic economy is for people that search on kayak/travelocity/whatever and sort by price.

Helps DL compete with the "low cost" airlines like Spirit, who otherwise would win at the pricing game every time (though they often still do, even if they turn out more costly once you add in all their "fees" for normal things that DL offers for free).

What I don't get is why DL won't let people that purchased E fares pay more after the fact for a seat assignment/upgrade privileges/etc. seems they could price it higher than the next availible fare bucket and make more $$!