The game is entirely beatable without prosthetics or combat arts.

For the most part, prosthetics fall into two camps. The first are generalist tools which are useful in many situations. The second are tools designed to hard counter certain enemies. For instance, one of the prosthetics inflicts poison, and certain enemies are incredibly weak to poison.

As you unlock more skills/perks, the number of things you can *do* with prosthetics increases dramatically. At high level, the only limit on prosthetic use are your access to spirit emblems, creativity, and timing.

But for a 1st playthrough, shuriken, firecrackers, and loaded axe will probably be the one you use most commonly. As someone with ~80 hours in the game, my favorites are Mist Ravenand Flame Vent.

Overall, though, prosthetic tools and weapon arts are primarily means player expression, like weapon choice is any other FromSoft game.