It differs from builder to builder. There are builders for MCPZ that work for other team and/or other servers. For example I do a lot of work for small servers that have the chance to grow. The reason I do that is because if they do grow well I just got my name out there. They also usually don't need much and I can give them a really good build to attract players to stay and think this is worth playing on.
Now to the duties well we build maps and lobbies for MCPZ. We are allowed to do work for other servers for commissions. MCPZ pays for our builder server and in return we build what ever they want. I personally do a lot of work with the admins when they need something fixed or changed. I am essentially teamplayers "Female Dog" (you know what I mean).
We build good and in different areas. I am HORRIBLE at organic but its also the hardest style to build with. Each builder is good at different things.