Static typing is great but not essential when the program is small and you can keep the model in your head, but the larger the program, the more you benefit from static typing. Static typing helps with tiny refactorings, but it truly shines when making non-trivial changes that don't play well with the existing model; it's as effortless as making the desired changes to the model, then mindlessly fixing all compile errors, and it will likely just work afterwards.