It is also-or was-slightly cheaper than having the two separate cards
http://gulfnews.com/news/gulf/uae/government/identity-card-can-be-used-as-e-gate-card-at-uae-airports-1.502831
<em>edited by wuffles on 12/04/2011</em>
You have to register your Emirates ID card at the e-gate counter at the airport. The great thing is that you can do this on the spot at the airport and your card is activated immediately. It's so convenient! At least I can justify having an Emirates ID card now!