why do we attached http profile to SSL vip?

HTTP profile is essentially a buffer+parser which checks whether the request format complies with the standard. When used, it opens up new possibilities for request/response adaption and payload modifications. It has some unintended benefits from the security standpoint too, for instance, it protects your web-servers from Slowloris attacks. So I would use it even when it's not a pre-requisite for your iRules or LTM policies.