Why no one tanks...

[Pseudopsia]

I've also had a tank who would use Convalescence before a pull and randomly cast cure on themselves for no reason.

I recall it was effective as a PLD tank in FFXI to cure yourself, this is why I think you see new players trying to do this. It's useless in FFXIV because rage of halone combo(single target) and flash(multi target) is superior for enmity generation.