The beauty of it is that you can have a better overall view of the protagonist. For example, if your protagonist has a specific point of view about themselves, you might swing around to another character who draws a completely different picture. It gives the reader food for thought. Maybe the impression that the protagonist is giving isn't completely accurate. Then again, it might also change your opinion of the secondary character as well, since you will be seeing them up close and personal.
I do this with almost every one of my books and find that it really gives me a different perspective too. Sometimes the only way to really understand where a secondary character is coming from is to climb inside of their mind. It's really fascinating.