You're suggesting there's no financial gain? :)
The majority of the time the virus is there to:
1. Make you download, and pay for, a product which will "make it go away"
2. Scrape details from your keystrokes/logins/files which gain the virus writer access to your bank or social accounts
3. Propagate itself.
The "kicks" which come from it may also include the "fun" of "beating" a security system.
Usually the best advice is: if you don't understand something, don't do it. :)