Because you need more than one player who can play in any position.
Marrs is a right-back, covering at left back. We only have 4 defenders signed at the moment, so any injuries and suspensions and we'll be moving Dixon/Roberts/Sheridan to full back with Mills to cover centre-back/left-back.
The tall centre back for Sunderland impressed me - maybe we can sort out a deal for him? Looking at their report, they had a couple of U18s in the back four... so they may be willing to loan them out?