Basically the central idea is that null pointer reference issues should get caught at compile time instead of run time. So if you are writing a function that takes some object and you don't want anyone to call it with null references then type system should allow you to specify that requirement so that compiler can use it to provide guarantee that call to your function would never compile if caller does not satisfy your requirement. This can be done in many ways, for example, decorating your types or using option types (also called nullable types in some languages) but obviously that's lot more work for compiler designers.

I think it is understandable why in 1960s and 70s, compiler designer didn't go all-in to implement this idea because machines were limited in resources, compilers needed to be kept relatively simple and no one really knew how bad this would turn out 40 years down the line.