[PeterisP]

One aspect is that at least in one company where I have wroked the job descriptions, skills and qualities were quite official and used everywhere (and actually reflected reality quite well) - so for hiring purposes, you get a list of what that person needs to [be able to] do, but that list is mainly written for the existing employees doing that exact job, or switching between roles within the company (external hires were a bit <50% IIRC).

Sure, you may not know X when you're hired, and it's not a hard requirement as you may learn it as you go, but it's on the list for a reason - you will need to know and use X on the job, you'll be evaluated on X during your semi-annual review, so X matters for the hiring ad / job description as if you don't fancy the idea of learning X and doing it for half of your working day, then I'd rather prefer if you know about the need for X before you choose to apply or not.