Fingerprinting an Operating System helps a cracker because:

A.
It defines exactly what software you have installed

B.
It opens a security-delayed window based on the port being scanned

C.
It doesn’t depend on the patches that have been applied to fix existing security holes

D.
It informs the cracker of which vulnerabilities he may be able to exploit on your system

Explanation:
When a cracker knows what OS and Services you use he also knows which exploits might work on your system. If he would have to try all possible exploits for all possible Operating Systems and Services it would take too long time and the possibility of being detected increases.