Decline Button

I've been wondering for a long time, what is the point of the decline option in matchmaking? You know how when you press the big PLAY button up top and when a match is found it shows the timer with either accept or decline? There is literally no point in declining. You pressed PLAY for a reason, why would you not press accept? Maybe I could understand if you don't like the teammates/comp that will be in the match but you can't even know that till after you've already accepted.

And yet there is occasionally that one guy who declines. So... why?

In one sentence: What's the point of declining since you just pressed PLAY?