All different reasons... sometimes, they just don't want to play anymore, and if they have a huge stack, they know simply blinding out will raise them a lot of positions in the tournament while others bust out.
Some people do it when they have an enormous amount of chips just to keep from getting in dumb situations before the final table. I disagree with their ideas wholeheartedly, but people are people.
Or sometimes they just have connection trouble and have no choice in the matter.
There definitely isn't really any strategic reason to ever do this, except possibly in the case of a satellite tournament if you think you already have enough to where you can't possibly miss out on the seat.