i drew it in midway, and i kept it in because i thought it brought an interesting balance to the piece. the first reason (obviously) would be that since Toph is blind, and blinding Sokka would open him up to her world a little bit, through touch and sound. A little bit deeper reason would be that for the majority of the first season, Toph tries to hide her feelings for Sokka from him as well as from herself, and he is completely oblivious to her affections. I thought it would be a nice touch for him to be blind when he finally 'sees' these emotions for the first time, and for them to be on something of an equal ground when she finally comes to grips with her true desires.
as for circumstance, i wish i had a cool fanfic or something to link to lol. perhaps sokka has been captured and blindfolded, and through some struggle, toph saves him.