Given that there is a highly likely chance that one of our two centerbacks will either fall out of form or suffer an injury (at least a minor one) at some point next year, I think it's pretty crucial to have a quality backup. It was a great problem to have from 2011 or so to 2014/15 when we always had 3 quality centerbacks (originally Heitinga, Distin, Jags) and then Stones, Distin, Jags competing for two spots. Think it made sure players played to their best and protected us from injuries.
Again, I think centerback is one of the most crucial positions on the pitch, so we need to be well covered. After signing a quality backup, Martinez needs to determine whether he thinks Galloway/Browning/Pennington would be ready to step in as an emergency 4th choice or if we need to go sign someone for cheap who can be emergency cover.
Can I just say how if there's one player that we've mistakenly let go over the past decade, it would appear that it's Shkodran Mustafi. Never got a look in here and 3 years later is a German national and is now linked with Real Madrid.