The problem a coffee bag has to solve
Two things are happening at once and they pull in opposite directions.
The coffee inside is giving off carbon dioxide for days after roasting. Seal it in a completely airtight bag and the bag inflates and eventually bursts. But the thing that actually stales coffee is oxygen, so venting it by leaving the bag open is worse. The valve solves both. Gas pushes out through it, air cannot get back in.
How to use the valve when you are buying
Press the bag gently and smell right at the valve.
If you get a strong, sweet, roasty smell, the coffee is still actively releasing gas and it is fresh. If you get very little, it has been sitting a while. And a bag with no valve at all was either filled long enough ago that degassing had already finished, which tells you something on its own, or it is going to swell on the shelf.
It takes two seconds and it is the single most useful thing you can do standing in front of a shelf of coffee.
The rest of the bag
The valve gets the attention but the material matters just as much. A coffee bag needs to block light, block oxygen and block moisture, which is why good ones are opaque and feel more substantial than they need to. A clear bag looks lovely and lets light at the coffee, which is why you rarely see one on a specialty shelf.
The resealable strip is doing real work too. Every time you open the bag you let air in, and getting it properly closed again is most of home coffee storage.
Try this
Do the valve test on every bag of coffee in your kitchen right now, including the ones that have been open a while. The difference between a fresh one and a tired one is obvious once you have smelled both back to back.