How necessary is it to fully purge old fluid during a transmission fluid flush?

I see a lot of these videos on YouTube of folks pouring in several liters of automatic transmission fluid into their transmissions as they collect the increasingly cleaner and cleaner fluid that displaces the old. They go some like 5 rounds till all the old stuff is purged as evidenced by the purity of the color coming out of the radiator or some other drain port.

I can see the logic of having unadulterated fluid in the transmission, but I’m curious what exactly is in a tranny that’s so sensitive to any colloidal debris remaining in the fluid. Is it that abrasive debris is attempting to be fully removed or that the fluid has lost some of its lubricating properties or some other desirable physical properties over time?

Basically why isn’t something like an oil change (drain and fill once) appropriate for transmission fluid?

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Here’s some examples I gathered of single pass vs multiple pass fluid changes.

Single Pass

Volvo S60 Transmission Fluid Replacement – Smooth Shifts! (S40, S60, S80, V70, XC70) Total of just under 4 liters drained from the tranny.

Multiple Pass

DIY Volvo automatic gearbox flush AW55-50, MANNOL 3309 with oil comparision
Total of 18 liters flushed from the tranny.

Volvo Transmission oil change Aisin TF-80SC. 6 speed Auto. Total of 12 liters flushed from the tranny.