Two reasons.

First, YJIT/ZJIT do much better when they know the type signatures of methods. You pay a performance penalty for implicit polymorphism, e.g. using a mix of types (Integer, Symbol, String) etc in the same method argument.

Second, from my experience with Typescript, as much as I naturally dislike type declarations, I find it does help LLMs. Having strongly typed libs/gems and being able to mix in untyped app code would be a nice balance.