It’s just so you start from known valid state. The only thing that actually needs to be done to put the phone into flashing mode is to hold Vol+ while it boots (or reboots).
The problems start when, say, your phone gets into a locked up state where the SoC is on and needs full reset, but it appears off to the user - so you need to hold the power button for ~16 seconds to reset it. BUT… because of the way full reset it implemented in the battery charger controller, it doesn’t work when there’s USB plugged in, so you need to plug it out. HOWEVER… when you plug it back in when there’s not enough juice in the battery, you may trigger a condition on early batches where USB-C port becomes unoperational until another full reset. This combined with the fact that there’s no indication that the phone went into flashing mode and it still appears off to the user can lead to a lot of confusion and walking in the dark trying to make it work.
So… it’s just easier to recommend taking your battery off even if you don’t really have to do it 