You don't have too, but it makes your visit much better, even if you have FOTL.
Here's a few advantages to arriving early, even with FOTL.
1. Front seat rides on the coasters.
2. Walk ways are less crowded until 11am.
3. Not as hot out.
4. FOTL waits are shorter.
Let's say you arrive at 11am like everyone else. You'll find a crowd at the gate, walkways will be packed, and the FOTL wait for the most popular rides will be 20 minutes.
I'd go to bed early and tell everyone you're waking up at 7am so you can get there at 8am. Come back to your room in the afternoon and nap.