Why do some conference have an "abstract submission" before the "paper submission"? I noticed that some conference have different deadline for paper submission: an "abstract" submission deadline, before the usual "paper" submission deadline.

For example, on the International Semantic Web Conference 2013 webpage you can read:


  Submission dates
  Abstracts: May 1, 2013
  Full Paper Submission: May 10, 2013


Why do they need the abstract before the paper?
To estimate how many papers they'll get?
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.