The brew is the unit of account. Malt, hops, fermentation time and final volume under one number are the only way to compare two runs of the same beer.
A keg is property out on the road. Not knowing which keg has slept eight months at which pub costs the value of a fermenter every year.
Deposits are an accounting system of their own. Keg, coupler and crate carry values that must return, and without an account per customer they do not.
The revenue office wants figures that already exist. Keeping produced and delivered volume per brew in order turns the return into a query instead of an evening.
The taproom and distribution share the same stock. A keg on the bar cannot be reserved for a customer at the same time.
Unfiltered beer has a short and unforgiving life. Tracking the packaging date batch by batch decides which beer leaves the warehouse first and which ends up coming back as a return.