The ability to sell specific stocks. Fidelity has this feature
I don’t see it documented on the site if stock sales have any guaranteed order (e.g. oldest first). Some countries have 0 capital gains if selling stocks after n years so it’s useful to guarantee the sale of the older ones. Similarly if you suddenly need some cash and the older stocks aren’t old enough but the most recent stocks are negative it would allow you to sell the more recent ones and avoid the CG due to it having a loss (and then wait the time period to sell the ones in profit).
People could sell a combination of older & newer stock purchases to balance out P/L too