Maybe its for when you don't have a spell component pouch or eschew materials. It can be hard to come across owl pellets or cats fur when in jail I guess. This looks best on a bard since he's a spontaneous caster and cast this in a pinch.
What's the purpose of Human Potential? Why would you cast it over any of the animal stats? This has to be the most useless spell in Pathfinder
It trades raw power for versatility.
Sure you can prepare or spend a spell known on Bull's Strength But Human Potential can be used if you need more dexterity, constitution, or even wisdom in that same day on short notice.
For a spontaneous caster this spell is even better, as they use one spell known to gain 6 different buffs. Though they are weaker than the specialized buffs.
Also the greater version is also nice in that each individual taget can choose it's own stat bonus. So the party members can decide on what stat they individually want higher.
Sadly it doesn't stack with a belt/headband but neither do the animal spells so that's not really against it.
Bards, sorcerers, and psychics are all spontaneous casters. If you have a party that uses most of the stats, and haven't gotten headbands/belts, this means you could buff anyone in the party by a bit instead of trying to learn all 6 different animal stat buff spells. For prepared casters, it means you don't have to prepare all 6 different animal buff stats.
As to most useless spell in Pathfinder, you compete with Malfunction, which only targets constructs but doesn't bypass construct immunity to effects that require fortitude saves, and golems are doubly immune because it also allows SR. Not to mention it is from the Transformation (not transmutation) school which means any school-specific benefits you may have do not apply.
Or you compete with Celestial healing, which ranges from being infinitely worse to being exactly as good as Infernal healing, considering it can only heal 1HP per 2CL. Yes, Infernal healing makes you evil, which can be a problem. Then again, being forced to become good also can be a problem. And both can be counteracted with a wand of the other, which amuses me to no end.