They are able to move across to cover the wide areas more quickly and more effectively than a single pivot would be able to.
The support and security provided by a double pivot allows the full-backs to push forward more effectively. They will often do so at the same, which may not be the case from a 4-3-3 formation. The defensive midfielders in a double pivot also protect the spaces between the lines and ahead of the centre-backs. They are able to move across to cover the wide areas more quickly and more effectively than a single pivot would be able to. This means they can get across following a switch of play, or cover the spaces an advanced full-back leaves vacant.