During peak times, the park opens early, but they don't post it, just happens. The official opening time is always 9am. During the busy summer months of July 4th thru August 20th, you can bet the park will open at 08:30am every morning. I've even walked up at 08:30 and people were already riding Hulk, which means the park was open at 08:15am.
At 08:30am a line forms at the entrance of what ever park you are visiting. At IOA the line goes from the gate past the ticket booths. Many people know about the arrive early trick and take advantage of it.
The reason it is such an advantage to be near the front, is because they don't have a rope drop like they do at Disney. In the past, they used to let everyone in and hold them at the bridge leading to Hulk, but now when you get into the front gate, you can go on Hulk immediately. This means you have a HUGE head start if you arrive the gate at 8am and you're first into the park. If you get there at 08:30am, you'll be near the back of the line, and you'll have to wait for everyone to go thru the gate. This doesn't sound like much, but it is, because people have strollers, sign tickets, and get stuck at the gate with tickets that won't scan. It's a real PITA standing behind people with problems while you watch everyone else go into the park.
If you arrive the parking garage at 8am, and the gate at 08:15am, it means you can take a front seat ride on Hulk, go again in the back, ride Dr. Doom, and Spiderman all before 9am. There have been times where I was on Hulk in the front with only one other person next to me. I have been on Dueling Dragons many times as the only person on the entire coaster. It's really something to have an entire coaster to yourself in the front row.
If the park doesn't open early, then it doesn't matter, you're still near the front of the gate and you'll get front seat rides on the coasters with no wait.