You can use a static analysis tool to check Ruby types ahead of time (still not a compiler) or provide information for tooling.
Alternatively you use them at runtime to check the correctness of data, which I don't think you can usually do with say Typescript where the typing information is for the most part compiled away[1].
1. I may be out of date on this but when I last looked at runtimes that could take Typescript directly they just threw the typing away. You just didn't need to use the tsc compiler first.
