I know that this is to clear cache, cookies, DNS and also other things that i don't know.
I want to know when should i "Create new identity".
Tor Stack Exchange is a question and answer site for researchers, developers, and users of Tor. It only takes a minute to sign up.Sign up to join this community
It depends on your privacy needs. Tor does this on it's own to an extent every ten minutes (at least with your exit node)
For instance, if you wanted to visit a site repeatedly as a new user, you should do it every refresh. For normal users, the default is fine. It all depends on what you feel you need. To completely start anew, every so often restarting tor browser is best