Why is there so little information on this game?

[Urthdigger]

Part of the problem is that the formulas are changed every expansion to keep things from getting too ridiculous as levels go up, so the game may have been out for 8 years but these numbers have been out for just a year. This is especially important to do for stats that become better as you get more and more of them, such your attack rate and crit chance. We know it's not a strictly linear scale, doubling a stat doesn't necessarily double the effect, and we do usually get some pretty good numbers about how stats perform, especially later in an expansion, but it's also largely irrelevant for most players.

Here's the short version of how stats work: "Potency" is just a stat used to compare abilities against each other. 100 is essentially a "baseline" and it should be noted that physical autoattacks and failed melee combos have 100 potency. They're not all that useful for comparing between classes (900 potency on a healer and 900 potency on a dps are two very different things), but they can reliably be used to compare between abilities.

Attack power, Healing Magic Potency, and Attack Magic Potency can largely be ignored. There's no way to directly increase these, so they're just derivatives of your other stats. You only have to worry about raising your main stat (which is STR for melee), your determination, critical, and direct hit rate secondary stats, either spell or skill speed depending on role, and tanks and healers can theoretically worry about tenacity or piety but in practice they're garbage. Your main stat is always the most important thing to raise, increase that at all costs, direct hit is generally pretty solid, determination is... ok, and critical is on a weird exponential curve where at lower levels it's not quite so useful but as you can stack more and more of it, it becomes better and better. Skill/spell speed are mostly used to hit certain GCD breakpoints for getting stuff into buff windows, as they're lower powered enough that the extra GCD or two they may give during a fight is not usually worth making the attacks themselves less powerful.

Lastly: In the example you gave, healing magic potency actually did NOTHING for Second Wind. That only impacts healing magic, which healers (and Arcanist/summoner) have. All other healing abilities, like Second Wind or Clemency, use attack power instead so they're not useless at endgame.

Edit: Also, really don't worry about this until endgame. As you're leveling, the only thing that matters is main stat, which will always be on the piece if it's suited for your class and will always go up as item level goes up. So, don't make bizarre decisions like wearing mage robes on a monk, and equip the highest item level gear you can, and you'll be fine until level 80.