The idea is to have gradual typing, like Python does. Most annotations in Python have no impact on runtime (e.g. parameter types aren't checked), but they are still valuable for external tools to perform type checking. A program with the wrong types will still run, but you will get type errors from the checker.

One difference is that Python specifies the type system in PEPs (basically Python's RFCs), while OP didn't ask for that here. This would mean whatever tool is implemented would define the type system.