Is using real world scale necessary, or just modeling everything proportionately to your characters?
Size is extremely important to animators. The size of the model determines how the animation will look. A building-sized ant-man will have different motion than a tiny-sized ant-man.
But in 3D the scale is relative, you can pretend 1 unit is 1.5m or 0.5foot or anything if everything adheres to it in the project and you don't care about anything outside of project and you communicate what the size is to the animator. But why to complicate life, just make 1 unit = 1m and be it self-explanatory. It's similar reason as to why name everything nicely in the scene - it will work the same as with bad naming, but the other person using that file will have so much easier time having everything self-explanatory named.
Having the scale to real-world units starts being really relevant when simulations come into play. Simulation engines do take size of objects into account. You cannot no longer just make it proportional and pretend the size is anything you want. And animators can do simulation stuff too.
If you have physics simulations scale does matter. Blender is designed to work with real world sizes and real world weights.
When using cycles, lights work also in real world scale, both in terms of intensity and falloff.