I know that every ARPG has many skills and you need to chose the ones you build needs. But in Sacred seems that some skills are useless for any purpose I can imagine, so I want to ask if anyone knows what are their original purpose, maybe they have one.

I will use the Seraphim as example, but each class has their skills which apparently aren't work as intended, or others skills do exactly the same and better.

Useful melee:
-Hard Hit (damage to single targets)
-Attack (damage crowds)

Useful spells:
-Archangel's Wrath (damage to single targets)
-Celestial Light (damage crowds)

Useful ranged:
-Energy Bolts (damage to single targets)
-BeeEffGee (damage to crowds)

Useful auras, although in this game it is tedious to be constantly casting (I would prefer a 'reservation' system as PoE, but ok, at least I understand for what are useful, even when I ends up never casting them)
-Rotating Blades
-Strength of faith
-Light Shield

Useful in general:
-Combat jump (travel skill)
-Conversion (not so useful, but I understand for what is intended)
-Irritation (not so useful again, but understandable)

Now the combat arts I don't have idea for what purpose would be useful:

-Light (you can see without this anyway, and the damage to undead is ridiculous)
-Combat Kick (Hard Hit seems the same, but it is way better)
-Multi Hit (Attack seems exactly the same, but better)
-Whirling Hit (again, a second clone of Attack?)
-Hunter Seeker (the most absurd IMO, low range, low damage, difficult to use, and every other ranged skills seems way better, not mention if you will be launching things, better to specialize in ranged weapons).

Does anyone know the usefulness of these latest combat arts?

As you said, their usefulness comes from specific builds. Some of the skills are outright better in general, and there are skills that have better potential but need to set your character up (thus sacrificing the generally good setup). Sacred don't let you respec your character sadly so this is a trial and error thing, or you can look for a guide. Also on higher level and skill/art level the different skills have different development, making one grow faster than the other. Some skills start out horrible but after a point they start getting more benefits from leveling up.

Yes, I know, maybe my post was too long, I was asking only for the lasts 5 skills which seems useless for all purposes. These:

-Light (you can see without this anyway, and the damage to undead is ridiculous)
-Combat Kick (Hard Hit seems the same, but it is way better)
-Multi Hit (Attack seems exactly the same, but better)
-Whirling Hit (again, a second clone of Attack?)
-Hunter Seeker (the most absurd IMO, low range, low damage, difficult to use, and every other ranged skills seems way better, not mention if you will be launching things, better to specialize in ranged weapons).

I see, I don't really used seraphim in Sacred 1. There is a dedicated wiki for the game, I'm sure there you can find the skills pros and cons.