Yes, especially for July, because it's summer, which is a more popular time to go to Avalon, you want to be sure you get the specific boat times that you want. I don't know if the Express has changed how they do things, but they used to require a credit card (or debit card) number and use that to book and hold the reservation, but they wouldn't charge you until you actually arrived at the boat terminal and got your tickets, so that way you could pay with cash or another card, etc. I don't think they require full payment up front.
Of course, a lot of folks don't know which boat times will work for them until the last minute, and so they don't like to make reservations. That's taking a risk that the times they want won't be available, which could make a big difference if they have a lot of tours and activities planned for the day on the Island. Then again, there are always no-shows or cancellations, so those folks who are hoping to get onto a fully booked boat may be in luck of a no-show or cancellation happens, but, again, summer is a busier time so it's taking a chance.