See headline. Is there a reason for a purge tower?
It’s used to build and stabilize filament pressure. Still far less used.
Yea man. When the nozzle has been sitting there for some length of time, it’s not ready to go into action. The prime tower does this on a part you don’t care about, so it can hit the part you do care about with a full filament path.
AH, OK.
Thanks guys. 
some free letters for the forum.
Why not just have the printer make poop waste? I tried running a multimaterial print w petg in the aux hotend and it didn’t make poop and only used the prime tower, leading to a bunch of petg boogers on the supports with some ending up in the main part
From my understanding, there is a common misconception. Purge is what goes out the back of the machine. As a couple other comments mentioned, the tower is a “prime tower” not a “purge tower” and is used to get the pressure in the hotend correct before continuing. That having been said, there are several videos about minimizing waste out there. Bambu is rather conservative with the amount of filament used in the priming. This can probably be reduced and a smaller tower can be used (that can be set in the advanced setting under “Others”).
Latest beta seems to have optimizations for the size of the prime tower FYI.