We've never really needed dining reservations at Universal.
Pretty sure you can get one the day before if it's a busy time.
At least that's how it was in the past. I'm not sure how covid has effected crowds and operations.
Pre COVID, family of 5 that tends to go at busy times, I think we made 1 reservation in all the times we went. And that was day of. That was only because someone in the party really wanted to go to a specific place. Otherwise, we're usually flexible enough that if the place we tried was too busy, we'd just go somewhere else.
In COVID times, we're down to only the 2 of us going and we ran into some long waits when we didn't reserve. Day of was usually fine, but day before was better. We still had a few times where we tried Toothsome, Vivo, and Cowfish and all had 1 hour+ waits, then we walked to NBC Grill or Red Oven and walked directly to a table. Party size, dates, timing, flexibility, weather, specific restaurant, and luck all play a role.
You can remove all of those variables by reserving a few days ahead, but then you're stuck eating at a certain place and time which means planning any park activities around that "Do we have time for one more ride, or should we just head over there now?"
The flip side is, you can avoid the potential of a long wait to eat. That 60-90 minutes you are waiting is 60-90 minutes you aren't riding a ride. That 60-90 minute wait could impact you twice - once while you wait, then it pushes your 'done with dinner' time out that same amount. That may impact your after dinner plans "If we wait 90 minutes, we're going to miss the Hogwarts show..."
Our next trip will be with 8 people the week between Christmas and New Years... I'm not looking forward to planning out those reservations.