TL;DR

RemoteFunctions / RemoteEvents more or less must be used with FilteringEnabled, but BindableFunctions aren't necessary. ModuleScripts are one alternative.

Info

BindableFunctions let two Scripts (or two LocalScripts on the same client) talk to each other with something that looks like a function.

You don't need to use BindableFunctions to communicate, you could also just use IntValue, NumberValue, , etc, objects if you wanted to, though that is much less structured and harder to sync. You could also use _G or shared but this is usually frowned upon.

ModuleScripts are often a simpler, cleaner solution. The communication isn't precisely the same -- it's in a sense "one way" (only one can call the other), but that can be easily designed around (passing in response functions, etc). It also can make things much cleaner and clearer.

BindableEvents are a slightly more useful feature that is fairly similar to BindableFunctions, but lets lots of different scripts listen to events happen.

With FilteringEnabled, it becomes necessary to use RemoteEvents and RemoteFunctions if you want a Script to talk to a LocalScript or vice-versa. These have an almost identical api to BindableEvents and BindableFunctions.