I wrote a bit about DI boxes in our recent article - Tips For Improving The Sound Of A DI Bass. That post talked about how to improve the sound if you recorded your bass guitar parts with a DI box. The assumption then was that DI boxes are undesirable but sometimes necessary for recording. But things are often very different in the live sound world.
Here is an article talking about the many advantages of using DI boxes when using a PA system for live performances: