The return type "void" means that no value is returned. If we capture the return value of such a void function, it will be Null, just like when no return value is specified and no return type is specified either.
func foo() -> void:
var result = some_object.foo()
print(str(result)) # will print "Null"
result = bar()
print(str(result)) # will print "Null" again
In other languages, trying to capture the return value of a void function would result in an error at compile time, but script languages can be a bit different and more flexible.
IMO using " -> void:" is a way to communicate that no return value is forthcoming. You also can't have a "return value" statement in such a function in GDScript.