"Advanced" (read: decades old) type systems. They provide tools like algebraic data types, generics, interfaces, row polymorphism and many others, but just those four allow expressing a gargantuan amount of invariants in the type system without sacrificing flexibility. Invariants that can be proven once and be maintained throughout years of code churn without repetitive runtime checks, heavy discipline or overly repetitive automated tests that attempt to poorly reimplement a type system.
They can be used not only as a way to let the compiler help you out in not making mistakes but also as an exploratory tool to test out many edge cases with quick feedback and as an informational aid to help quickly introduce new developers into a codebase with its own business domain explained in a language that allows conveying "this is possible" and "this isn't possible" much more quickly than reading tons of procedural code until the reader comprehends all the implicit rules laid down by the program's behavior.
They can be used not only as a way to let the compiler help you out in not making mistakes but also as an exploratory tool to test out many edge cases with quick feedback and as an informational aid to help quickly introduce new developers into a codebase with its own business domain explained in a language that allows conveying "this is possible" and "this isn't possible" much more quickly than reading tons of procedural code until the reader comprehends all the implicit rules laid down by the program's behavior.