Why do some conference have an "abstract submission" before the "paper submission"?
From what I have observed, having a specific deadline for abstracts is used for two main reasons: having a rough idea of the number of submissions and organise a bidding for the reviewers. 

Having the number of submissions can help deciding of a possible deadline extension and possible to "recruit" more PC members or reviewers if the number largely exceeds the expectation. 

Organising a bidding based on the abstract allow the PC members to indicate their preference for each paper (e.g., I want to review this paper, I could review this paper, I couldn't review this paper), so that when the actual papers arrive, the distribution is already organised.