The main purpose of a DI is to balance the signal so you can send it to a mic input over a distance of more than around six feet without inducing hum or other interference.

However, it is also very useful (as Stu just said) for impedance matching, as most have a 0dB, 20dB and 40dB pad switch, plus an earth lift switch.

Please note that some guitar transducers don't like transformer balanced ('passive') DI boxes, and prefer a good quality electronic version. Passive boxes can make them sound thin and nasty, with reduced level.

Using a DI also removes the problem of shorting the Phantom Power when putting an unbalanced line into an XLR input on a desk where Phantom is global (as opposed to switchable on each individual channel).