What is the use of Login Window option in Mac. If user is already logged in, then what is the point of this option. When I select this option, it went to login window. Is it only to lock the Mac?
Well, locking the Mac is certainly one reason for it, and a good one.
Another reason would be if you (or someone else) needed to also log into another account without first logging out of the current one.
From your screenshot it appears that you do not have fast user switching enabled (probably because you may be the only user), but even if you had multiple user accounts and did have it enabled, the Login Window option would still be there as a way of locking your computer.