A cookie can be read by client-side scripts easily if cookie properties are not set properly. By allowing cookies to be read by the client-side scripts, information such as session identifiers could be compromised and used by an attacker who intercepts the cookie. Setting cookie properties (i.e. HttpOnly property) to disallow client-side scripts from reading cookies better protects the information inside the cookie.
As of 21.2.1, all cookies within Performance Management contain the HttpOnly property out of the box