By turning the fullbacks into centre mids when attacking, the actual centre mids are able to push further forward without leaving space behind them. The team effectively has 5 players occupying the centre and half spaces in midfield. The DM, the 2 fullbacks, the the 2 CMs.
Setting the team up this way allow space to be covered more effectively. The team is so equally spread out in every direction that it makes it harder for the opponent to shift to one side when defending. It creates more space for everybody, but especially out wide. Because the opponent's DMs and wide midfielders MUST guard the centre of the pitch and can't double up on the the winger, leaving him more space to go 1v1 against the fullback.