It's less "need" and "does the same thing but far more efficiently and can't rot because a machine makes sure it's correct".
I like letting my tools do stuff for me. If a tool can go look up what arguments this function expects, and their types, and tell me that, rather than my having to go check, and then also warn me if I typo one of them, then also take me straight to the definition of the function if I decide I want to read it, without my having to go hunt it down manually, all at zero cost (again, these things should be documented anyway), then why the hell not?
[EDIT] I guess I just don't understand why people consider it burdensome. You probably ought to know your function's going to take a string. Is it that hard to type "string" in the signature? You're gonna have to figure out what it's supposed to return—when you do, write it down. It's a little more work for more complex structures but it still amounts to typing out things you already have to think through anyway, and the more complex the more important it'd be to record that somewhere, so may as well be machine-readable type information. In return, when you accidentally try to treat your string like a number you'll immediately know you've missed a step, you'll get comprehensive autocomplete on hashes and classes and such, jump-to-definition, instant typo-spotting, automatic refactoring, and so on, and you can hand it over to someone else and they'll get those same things, instantly. The payoff is 10x, at least.