I'm planning ADRs for every day because it's more relaxing.
Even with ADRs you're sometimes kept waiting to get a table ... without an ADR, it might be ok for some restaurants, for some others you need to rely on luck, or be willing to wait a long long time, or possibly end up at the first place with availability when your stomach can no longer wait.
I've been turned down quite a few times when trying to do walk ins ... some restaurant in DTD told me I could get a table in 2 hours ... no way Jose.
I plan those ADRs depending on where I'll most likely be on that day ( depending on park crowds) but I will gladly cancel the ADR for the next day (no penalty if done before 10pm) if I decide to change my plan for that next day.
You can go without ADR, some do that and are pretty happy about it.
I make ADRs because I want to be sure to be able to eat what it like, at a "civilized" time.
It all depends on when you go (crowd wise) and at what time you want to eat (trying to walk in at 12:30 for lunch is often a recipe for failure, as well as trying walk ins for sunday morning) and what you want to eat.
Even if most of the time you'll be successful finding a decent restaurant, without a big wait (half an hour you can expect) also bear in mind that you might end up at counter service eating a hamburger when it would have felt nice to seat down and enjoy a meal
at the end of the day, you'll see what suits you best.
Also remember that you can drop by guest relations, or at your front desk (if you stay onsite) or even go online, to try and make reservations for the days to come.
In the end it's always better to try and make an ADR, even if it's for the next day, if you fancy going some place that next day. If there is availability you'll secure your table.