The DI makes it safe to run an instrument signal into another machine. Most come with add ons like sims, ground loop cancel, pads, balancing signal etc. Almost all decent effects units will do the same thing + more BUT NOT effects pedals unless they state otherwise as they usuall UP the signal, noise etc etc. Folk use a DI often to get the sound of their guitar/bass without the colouration of an amp & speakers or where the use of amp/speakers makes mic recording difficult.
The best analogy I can think of is that it is the difference between plugging the line out of a CD player into the line in of an good stereo. The levels are correct for the stereo to work with.