The primary purpose of cookies is to store client information for server. Without cookies server would have to deal with following problems:
1. Identify the client. Signing in at every possible web site is impossible. Identification via other means (user agent, screen resolution, canvas fingerprint etc.) might identify the client, but not 100% reliable.
2. Store some information about the client. It was more important at the beginning of Internet. Now storage is relatively cheap and for many web sites the amount of storage for cookies even for millions of clients is not a big problem.
That is why in a application with good architecture client logic does not depend on cookies contents and thus client does not need decryption of cookies.
Encrypted cookies give normally false feeling of security. When an attacker gets access to your file with cookies, he can copy them and use in the browser. Server will receive correct cookies, decrypt them an will not be able to distinguish, if cookies come from you or from attacker.
Encrypted cookies have a huge disadvantage, because they make for user impossible to check what information is saved in the cookies. Users think that the web site is collecting some information that the users might not like and makes this information unreadable for user. Thus encrypted cookies make web sites suspicious.