It can also be just to fill rolls though. I mean technically anyone does it whenever there's any person a character has to talk to who is not another's character (ex: Tavern lady, shop keeper, the bandit you just fought etc.) but what most people refer to when they say double is really just that made to be a more plot relevant character, which is almost necessary in all RP's, especially 1x1.

I can think of one 1x1 particular I was in (where I believe even at the day the Old Guild died it was the 1x1 with the most amount of posts in it, so obviously a lot of plot was needed) where new characters where constantly made to help with the plot. My main's father who acted as a villain for a while, the other player was playing a kind of Assassin whose father did the same and a lot of her father's old friends were roleplayed to act as Guides/supports to for the battle's and challenges that were to come. 

Also if our characters were to ever be in different area's for a reason (at one point we had to split up for several months) we needed other characters for them to engage if for nothing more than to keep them engaged, have more stuff to post on and also simply prevent them from coming back insane from a lack of social interaction.