Consistency makes code predictable and reduces mental overhead. It doesn't mean you have to write it poorly like the rest of the codebase, but it does mean using the same general practices as the rest of the codebase. Think of it like using knockoff legos vs the real thing. They both work interchangeably which makes it easy to use them together, but you'd prefer to use the nicer lego pieces as much as possible in your builds because the material is higher quality, tighter tolerances, just overall works better even if it's the same shape as the knockoff pieces.