One reason for a timed essay on the SAT is that they know reasonably well that YOU DID IT YOURSELF -- no help from parents, "admission counselors," paid prep aides, etc. Second, in real life, as well as on College exams, you frequently have to think, write and produce a piece of writing on deadline. It's not the only thing, but most folks who can't produce a reasonable piece of writing in 30 minutes aren't going to give you Paradise Lost if you give them all year!