I was reading that it was possible to clean a movement plate with the balance installed, but the balance jewels needed to be removed first.
Can someone tell me why the jewels must come out? I assume it is because of the pivot points...

Michael,

As with all this work there are very few hard and fast rules, under certain situations and types of WW movements or modern style PW with incabloc mountains you can clean with the balance held in place by plates and cock, with the upper and lower jewels removed, (those being set aside for hand cleaning/oiling) you would of course have to make sure the movement is secure in holder and no possibility of the movement sliding or other part in the basket hitting the balance and damaging pivots/hairspring during the cleaning cycle.