Sometimes, in RP posts, you may want to include an extra person you can have on your side, but want to play yourself.
For instance, me and my daughter
I can RP as me, and have Melody beside me and control her actions.
All an NPC is, is practically a way to further expand ones abilities and length of a post.
I can do three para's as just me, but if I throw in Melody, I could double that xD Or triple, because it would give Sakura an extra person to talk to, meaning I could post more as her. Get it? <3