With a hash it's difficult to find a combination that results in this specific hashed password. Think of it like this: you have a biiig prime number and you multiply it by another. Now, that's easy, but it's way harder to do it backwards - factorize a large composite number (this is just for illustration). Similarly trying to find a password that works when you input it based on the hashed one is way more difficult than hashing the password in the first place.