Speaking of, here is an exercise for you: why must the locktime be an absolute number? Why can't it be a relative number?
Uhm...
I would think it is because you can never be sure which block will carry your transaction. I guess an absolute number gives you an approximate date. Roughly, you can imagine a block will appear each ten minutes and the transaction will be accepted around the time you include in the timestamp.
That's exactly it. You might fight this post by satoshi on that subject interesting: