Wow! I know the answer to a "cooking" question!
Been a while since I made a pot of tea with loose leaf tea, but................"one for the pot" is because you don't want to drain the teapot completely for the last cuppa, so as well as extra tea you add.........extra water! - so you intentionally make and then waste one cuppa cha (no matter the size of the teapot).
Why? So that any impurities / tea dust within the tea leaves settles to the bottom of the pot and are not then decanted into the strainer - being smaller than tea leaves would otherwise then end up in the cup. Plus that last cup will not be 50% leaves when coming out of the spout. The rounded shape at the bottom of a teapot acts as a "sump" for both tea and impurties........plus a tea strainer full of tea leaves makes getting the last of the tea into the teacup slower / a PITA.
Historically probably more important due to both less consistent and intentionally varying tea qualities, coupled with strainers pre the nowadays more usual fine plastic mesh being not so good at both straining out debris and also the tea leaves.
If you don't put in "one for the pot" its the same as having a teabag cuppa, but topped up with dust from the teabag box - I don't think anyone ever puts that in
Why waste 1 whole cup of tea? because it's easy to measure!.........plus it appeases the tea gods
.....that's why squeezing too many last top ups from the pot has the tea tasting a bit funny - the tea gods don't like pilfering
PS. I know me first post should be an intro - but I got all excited to know a cooking answer . Intro to follow