A spectator thread is a thread that is created to watch another thread. This is usually done when players are watching something that they cannot interface with, such as a dueling match or a quidditch game. A spectator thread is ongoing at the same time as the thread it is linked to so a player cannot post in both threads because they can only be in one place. 

Why is this used if I cannot interact with the thread I am watching?

• This is so you can react to what is happening.
• This is so you can show moral support.
• This is so you have a different environment to RP your character in as you explore your character.

Can I use my abilities to help out my favored team?

No, you cannot for instance help them find the snitch because you have an ability that grants you some sort of advantage in doing so.