It is recommended that you make a PS for any restaurant that you REALLY want to eat at AT a certain time especially if it's during a holiday or peak season.
Like the previous posters said it is not a reservation but if you don't have a PS and you try to go to a specific restaurant (one of the more popular ones)...you may be told they can't take any walk ups which would be anyone without a PS.
This past trip we just came back from, I had a tough time getting PSs at any of the places I really wanted to go to at the times I wanted, however we did end up going to one place without a PS and it was no problem being seated. It wasn't one of the more popular restaurants though & was right after a big storm came through.
Places like Chef Mickeys, 1900 Pk Fare dinner and O'Hana's would be places you would need a PS IF you want to eat at a certain time. I know people who have had PSs for these restaurants and waited a while. And, I've known some who didn't have a PS that got in, just depends on when.
We had a PS on Sat. night for GG in Epcot and we waited about 25 minutes to sit, so you never know, just make them, you can always cancel

Unfortunately I had to reschedule our whole trip and that's what messed my PSs but we still ended up with some good places to eat at with minimal or no waits.