They are supposed to be super luxury ergonomically designed socks, with toe bits at the front just the right shape for the human foot, hence the need for L & R demarkation. The only problem is one can't equalise big toe hole wear by swapping them to opposing feet.
As Dilbert said, "gift vouchers; like money, just not as useful". M&S products are like gift vouchers; buy 'em something, and if they don't like it, they can exchange it, but only for more M&S stuff.