Let me start by saying that miniatures are awesome. They are a great aid for both the Dungeon Master and the players, as they make fights and scenarios more tangible. You can see the huge dragon in front of you and can be actually scared by it.
Having said that, they are completely optional. And I’m not saying that in a “theater of the mind is better” kind of way (which for some groups may absolutely be the case). They are optional because there are alternative methods to represent monsters and heroes on a grid that are a lot cheaper. I’m talking about tokens.
Will they have the same effect as miniatures on the table? Probably not. Will they absolutely do the job and look cool while not breaking the bank? Yes!