#4 Covering wingbacks and counterattacks
The full-back position has epitomised the evolution of football in the 21st century. With players getting fitter, faster and stronger than ever, a full-back is expected to make telling contributions at both ends of the pitch. With more and more inverted wingers coming through the ranks, full-backs are expected to provide the attacking width.
In a two defensive midfielder formation, such as a 4-2-3-1, the two midfielders are entrusted the task of covering their wingbacks during their occasional forays up the pitch. The Everton team of 2013-14 used this tactic to devastating effect - Barry and McCarthy covered Coleman and Baines, and the latter duo racked up goals and assists.
In a lone defensive midfielder formation, the centre-backs split when the full backs venture forward, and the defensive midfielder drops in as the third example. An example of this case is Fernandinho's role when Manchester City plays a four-man defence.
In either of these cases, the presence of the defensive midfielder allows the wingbacks to play with freedom and protects the team from being exposed to sudden counterattacks.