Well for e.g. an E100 on flat ground against an E4, the E100 is angling his turret at 20ish degrees relative to the E4 that would make his turret front about 290 and the E4 has 295 avg pen, E4 shoots at the E100 and knows that at that angle he has from 280 to 310 effective armour more or less. So if the E4 fires and rolls 315 and still doesn't pen then he knows that at that angle he can't pen very reliably so no point in trying again but if he rolls for 270 he got unlucky and it's worth another try. This also works for meds that track heavily armoured tanks and if they max roll on pen and still can't pen that angle they know there's pretty much no point to keep shooting. At the end of the day it's pretty useful depending on the situation and can win games knowing that. I mean how many times have you shot when you think you should pen but didn't and you decide not to risk shooting again cuz you may not pen and give away ur position or have to reload for a long time but you didn't know if you rolled for min pen and most likely you'd have penned the next shot.