Also, some people (like me) will make a SS out of a 5/6/7 speed bike with freewheel. Those hubs are spaced so that they are towards the non-drive side so they can fit all the cogs they have. Well when you only have one, all of a sudden there's a lot of space left over and thus respace.
Once you've respaced you're rim is still in the same position relative to the hub, but since your hub has moved relative to the bike so has the rim, but the rim needs to be centered in the bike and thus redishing.