It provides a static (compile time) check that boolType satisfies the ValueConverter interface. The _ used as a name of the variable tells the compiler to effectively discard the RHS value, but to type-check it and evaluate it if it has any side effects, but the anonymous variable per se doesn’t take any process space.
It is a handy construct when developing and the method set of an interface and/or the methods implemented by a type are frequently changed. The construct serves as a guard against forgetting to match the method sets of a type and of an interface where the intent is to have them compatible. It effectively prevents to go install a broken (intermediate) version with such omission.