Left footed players tend to prefer the left naturally, same for the right. If you can use both feet you wouldn't care.
Colo who is right footed but can use his left very well, plays left of the centre for us. Williamson who left foot is poor plays right as this helps him. Same with Saylor who is right footed with a weaker left foot.
Not that hard to work out is it ?
It's not that simple though, is it?
John Terry has insisted on playing on the left side his entire career despite being right footed. People seem to be against the idea of us signing a left sided centre half because Colo plays on the left, even though he is more comfortable on his right foot. There'll be more examples, I'm sure.