In my experience, it's one thing to plan a trip months out, and a different thing altogether once you're there. People drop and move reservations a lot, and I got same-day and next-day SciFi ADRs last November.
It's the same thing with FastPasses, you never know when something will pop up if you check the app while you're at the park. It was a particularly successful day, rode SDD early, got a same-day Alien FP while chilling at Woody's, then a same-day Midway Mania after using our ToT and R&R FPs, which we killed time for at our same-day SciFi "A"DR.