If you have M customer complaints, and each one risks a differently-sized N customers... you better try to triage that vs just playing whack-a-mole with whatever comes to a random engineer first. I've never seen engineers plow through a bunch of 0-or-1-customers-would-actually-churn-over-this papercuts because it was easy and it feels good - the customer mentioned it! i fixed it! - while ignoring larger showstoppers that are major customer acquisition and retention barriers.
Nothing is knowable in only the same way that plans are useless but planning is essential.
"the biggest impact" isn't knowable so a bird in hand is worth more than whatever might be in the bush