2) The wing-backs stretch teams so effectively
The front three of Chelsea provide a lot of width as it is, and having three at the back means that when on the attack, Chelsea can commit an extra man forward. Whether this is a man from central midfield or a wing-back, the opposition defence can get overloaded and stretched.
A simple cross-field ball can open up some defences when Chelsea overload defences. The full-backs are forced to come in and adopt a more central position to help the centre-backs, and more often than not, it is one of the wing-backs that is left free.
A perfect example of this was last week at Stamford Bridge against Tottenham. Diego Costa was the one to offer the width for Chelsea on this occasion, left-back Kevin Wimmer was forced to come in and help Jan Vertonghen. This left Victor Moses free at the back post to win the three points for Chelsea. I said the wing-backs are key to this formation. They can provide defensive solidity in a back five, or they can offer even more width going forward as an attacking threat.