The beach ball springs into action when the system deems that it's waiting 'too long' for something to happen. It may be that the current setting is a little twitchy, and it's a little too eager to get in your face.
As to the reasons why... well, they are boundless. For example, yesterday I was preparing to upload a photobook for purchase in Aperture, and a beach ball appeared. Why? Because the program was attempting to communicate with an Apple server, and the server wasn't responding in a timely fashion.
Saving that file to disk, and you happen to have a drive that's reporting physical errors? You're going to get a beach ball.
Creating a new iCloud document and your ISP is having a spot of bother? You're going to get a beach ball.
Spotlight is working dead hard at indexing your file system, and some other processes are aggressively working with files? You're going to get a beach ball.
There's probably a setting somewhere that tells your computer how long it should wait before activating the beach ball.
As for myself, it doesn't happen on my computer that often, and when it does, I'm glad to know that the system is waiting on something to happen in the background.