This question is one that many people ask when thinking about hiring a DJ for the first time.  The obvious answer is "To have music at my _______ (fill in the blank), of course!"  But there are many other answers, depending on what kind of event you're planning; and they don't all pertain to music.
 

For a wedding and/or reception, there are several good reasons, many having nothing to do with music at all:

A good DJ will have microphones, and know how to use them, to help your guests hear what's going on.  (If your wedding is in a church, he should be a good enough sound tech to operate the church's system, should there not be someone available from the church to do it.)

He'll have a good selection of music suitable for the different parts of the ceremony, to create just the right mood.

He should be familiar enough with weddings to be able to help organize & coordinate it for you, without the need to hire an additional professional; of course, this means he'll be there for the rehearsal as well.

He can also MC your reception, and coordinate it, too, if need be.

He'll work with the caterers, photographers, and others to keep your reception running smoothly.  (He may even be able to recommend some of these other professionals for your event.)

Above all, he'll meet with you ahead of time (as much as needed) to determine your desires for this very special day, from what music you want to how you'd like the sequence of events to unfold.
 

For other functions, there are as many reasons as there are kinds of DJs.  Some specialize in interactivity - they're at their best when doing games, teaching dances, entertaining the kids, and generally making your party FUN!  Then there's the karaoke DJ (KJ for short) - he's pretty self-explanatory.  Others do teen dances, bar/bat mitzvahs, picnics, kiddie parties (that takes a really special kind of person), corporate functions, you name it - if there's a special kind of event, there's probably a DJ somewhere who's done a lot of them, and is good at doing them!

There's even the Christian DJ; he specializes in functions where Christian music is desired, since most non-Christian DJs know little about this category.  Christian DJs will often do non-Christian events, also, but without the questionable, suggestive, or otherwise objectionable music so common today.  (One of my specialties - clean music, that is.)
 

So, go ahead - hire a DJ.  (Pick me!  Pick me!)