You wouldn't, but since there is no refund...if you find yourself need to change your plans (in terms of luggage) then you are stuck as the ticket is non-refundable.
I have liked BA less and less in the past few years. The most spectacular one was during my UK -> Japan -> Korea -> back to UK trip. On the last day coming back I got to Seoul airport and tried to check in my bag, the lady said "you don't have any checked bag allowance.
I asked "that can't be right, I had checked in this bag to Japan and again Japan to Korea." It's the same ticket...it turns out that I had 2 bags allownace on the way out, 1 bag in the middle flight and Zero on the return leg.
I was soooo baffled, part angry but that was what their system said at the check in bag counter. It was then £90 or something to add a bag or £150 to upgrade to Pre Eco which gets me that bag allowance.
I upgraded in the end but i still could not understand how a ticket can give you different bag allowance, along the trip, going down too.