Hi and welcome.
It would be helpful to know when you are going to answer if you absolutely need reservations. But to be honest, yes they are necessary.
Table service restaurants book quickly, especially during peak travel and free dining times. So if you want a sit down meal you will have to plan in advance and book it.
Most places don't penalize you except if you are going to a place that needs a credit card hold....like CRT (cinderella's royal table, hoop de do revue), then if you don't show they will charge you anyway.
Counter service places don't require a adr, but again during peak and free dining they can be packed at meal times. So eating an hour before or after the "meal time" helps that.
We often encounter signs at the parks at midday that state, no TS restaurant has room, in order to eat you will have to use CS or go somewhere else.
So to avoid this I always recommend making your adr's. If you don't know 180 days out, try a month or 2 out, but know that most the popular places will most likely be booked.
Have fun on your trip and good luck.