Going so near the crease makes no-balls more likely.
Disadvantages : A penalty run is given away. Maybe more runs if the batsman can hit it. Maybe more runs in the next penalty ball. In 20-20 format, a free-hit also gives away more runs. Wickets (catch, stumping, etc) might be potentially taken, but the effort will be lost, only because of the no-ball. (( Case In Point : http://www.icc-cricket.com/world-t20/match/worldt20-2016/34 , ICC World Twenty20 2016 Semi-Final 2 , March 31st 2016 , Wankhede Stadium , West Indies won by 7 wickets ))
Advantages : None that I can think of. One very unlikely possibility was that the bowler wants to maximise the speed for the batsman, or minimise the Distance for the ball to travel. But if this is a real advantage, then there should be no batsmen batting from outside the crease. More-over, batsmen would not be charging down towards the ball. Hence this advantage does not sound correct.
With all these Disadvantages, why do bowlers go so near the crease ?
One Quote by Dhoni after the Match given above, where I have highlighted the point of this question :
"It was a bad toss to lose, as the dew started to come in for us. The spinner did not have much to offer with that and we have history about our spinners bowling with the dew. I am not happy with the no balls but other than that the boys played very well. We wanted the fast bowlers to make some sort of a game, they bowled decently but the conditions were tough for them."