One thing I'm not seeing mentioned here is anti-virus scan of files being opened and anti-virus scanning of the system. There's also things happening like monitoring programs to see if they display any malware like behavior.

There are other system utilities that eat cpu time. Backup programs that do compression and possibly encryption. Defragment programs, many of which now have options to monitor new files being written and defragment them just after they're written.

There's a lot of stuff running in the background that can eat a lot of CPU occasionally, especially during program load.

Don't forget that as more security is added, more programs that talk to the internet are doing a lot of encryption and compression. Sometimes even dual encryption where files that are to be stored on the server/cloud are encrypted in case someone hacks the server and gets the file, and the encrypted files are encrypted again by the transfer program - browser, sftp, etc.

For those of you who say that mobile devices don't need to use as much cpu, google "android security problems" and read some stories like

http://www.welivesecurity.com/2013/03/11/android-security-issues-does-a-microsoft-windows-analogy-make-sense/

http://en.wikipedia.org/wiki/Android_%28operating_system%29#Security_and_privacy

http://readwrite.com/2013/03/04/in-the-security-world-android-is-the-new-windows

http://www.nbcnews.com/technology/over-half-android-phones-have-security-flaws-research-finds-999197

David Ball