Essentially playing out from the back can give you two options if teams are good at it.
Teams will play it around the back, CB to CB back to CB, back to goalkeeper which invites the opposition to push up, eventually the CF will press one of the CB's or GK. The aim here is to try to commit the opposing teams striker's, no10 or ss, and their wingers to be within 35 yards of your goal. This is why the top teams insist on having a GK good with his feet if this is their tactic as the GK will then be the main catalyst for the attack. The new modern era GK can beat a line or two depending on how far the opposition have pushed up and drop it onto the LB/RB's, CM's, wingers toe which allows a quick transition where they out number the opponents going forward. We even seen this with DDG under LVG, we would play it round the back quite frequently in the big games. Once the opposition started their press, Fellaini would push up to join our attacking third and De Gea would punt it up to him to start an attack. Obviously their is more success to it when your GK is top class with his feet, and their CB's can play it round more successfully hoping to force the opposition to press.
If opposition teams play in a compact, high block but allow the ball to move from CB to CB without any pressure until the ball is moved forward then this is where having a good playing CB is a major weapon. As they are granted more time on the ball, they can pick a switch over the top of the full-backs head for the winger to run onto or they can hit the 10/ most advanced midfielder with a ground ball and the team is now in the attacking transition.
There are many factors as to why it fails. Firstly it takes a huge amount of coaching as to when to initiate the attack, and where the players should position themselves and not something that happens overnight as seen by City failing in their first year under Pep. It also takes good personnel for it to be an effective tactic, essentially you are asking your two CB's and GK to be able to keep the ball between themselves for sometimes 6-7 passes in a small area whilst getting closed down. You need your defenders and GK to be good on the ball and have the confidence to do it.