Is encrypting session id (or other authenticate value) in cookie useful at all?

In web development, when session state is enabled, a session id is stored in cookie(in cookieless mode, query string will be used instead). In asp.net, the session id is encrypted automatically. There are plenty of topics on the internet regarding how you should encrypt your cookie, including session id. I can understand why you want to encrypt private info such as DOB, but any private info should not be stored in cookie at first place. So for other cookie values such as session id, what is the purpose encryption? Does it add security at all? no matter how you secure it, it will be sent back to server for decryption.

How to set asp.net Identity cookies expires time

I use Asp.Net Identity to control my app’s authorization. Now, I need to do this: if the user does not operate in 30 minutes, jump to the login page, when he login does not select “isPersistent” checkbox.
And, if he selected “isPersistent” checkbox, set the expiration date of cookie for 14 days.
I try to do this by change the Startup.Auth.cs like this: