First of all the % of goals scored from a corner is quite low, in 2012-2013 in the premiership it was just over 13%. Here is a good article on why Swindon (considered to be proficient at short corners) use the short corner.
Some of the reasons why they use them is to make the corner routine unpredictable, Barca sometimes do this.
Corners are clearly tricky to convert into goals – the evidence is
clear – so the need is for something different, and that is precisely
what short-corners provide. They stop the opposition relying on the
same tactics, the same positioning and the same scouting reports.
Whatever the fans’ opinion on the use of short corners, it is the
variety that they give which allows any corner to work – short or
long. Like the spin bowler who occasionally delivers one that doesn’t
turn, short corners make the more obvious, more direct deliveries
dangerous. Short corners draw markers out of the box while longer
crosses drive defenders back leaving space for the short corner; it’s
set piece tug of way, pulling the opposition this way then that.
Another reason why Barca use this tactic is to keep possession, if the team is not good at corners or unlikely to score against a team who is good at defending corners then it could be wise to keep possession and try work the ball in the box or try a different angle of attack. Defending teams also have the upper hand on corners because they are allowed to get away with so much fouling etc in the box, the fact that keepers are so well protected by refs and the mix of zonal and man marking from corners.
Basically some teams use short corners to try and confuse the defending team who may have been drilled to defend against a certain type of delivery that the attacking team has used in the past, however corners don't tend to lead to a large number of goals unless you've got someone like Drogba attacking the near post.