#4 Restore balance to Barcelona's side
Fans of the Catalan giants know that the only reason they managed to make La Liga a close run contest in 2016/17 was Messi's magical ability to constantly bail them out at the death. Luis Enrique's last season at the club produced a woefully inconsistent side that struggled to balance their attacking forays due to an almost non-existent right side.
Messi realised the problem and dropped deep, but that switch came too late into the season and confused Ivan Rakitic no end - as the Croatian was often unsure when Messi would drop and when he would stay forward. Going by Barcelona's signing's of Gerard Deulofeu and Nelson Semedo, it's quite clear that they've solved the issue of attacking from the right wing.
Messi can drop into midfield and dictate play to either wing, thus confusing opposing defences as to which side they can anticipate an overload. His ability to retain possession under pressure might also afford Barcelona the chance to play a more patient style of play, as opposed to the direct side we saw under Luis Enrique. Ernesto Valverde was heavily endorsed by Pep Guardiola, and with Messi in midfield, we might witness Tiki-taka v2.