Here is my perspective.
Add New Track
I use it in Grave Robber. When un-nesting nested sequences, Grave Robber accounts for possible clip conflict. If during un-nest existing clips will be overwritten, Grave Robber will create new tracks.
Get/Set Track Targeted
I presume these are the methods that @remco is talking about:
track.isTargeted()
track.setTargeted()
They are part of ExtendScript API. I use them in my extensions extensively, to allow users to target tracks where extension operations will take place.