You're still in charge of scheduling when you'll actually get the work done.

The benefit for the client is that they don't need to review and sign a brand new Statement of Work whenever they want to change a button's color or tweak a feature.

Should your client need work that will seriously disrupt your schedule / happiness... I wrote my thoughts on that a few months back: http://planscope.io/blog/should-you-charge-more-for-a-rush-j...