With the explosive growth and distributed nature of computer networks, it has become progressively more difficult to manage, secure, and identify Internet devices. An outsider has the capability to discover general information, such as which operating system a host is running, by searching for default stack parameters, ambiguities in IETF RFCs or non-compliant TCP/IP implementations in responses to malformed requests. By pinpointing the exact OS of a host, an attacker can launch an educated and precise attack against a target machine.
There are lot of reasons to hide your OS to the entire world: