To be entirely honest, I always viewed Arcana as the practical skill to understand magic. Not Magic Theory, which is WAGs galore, and spouts things like the Unified Magic Theory without any proofs being done. Or, in another terms, Arcana is the engineering version of magic(how it actually works), while Magic Theory is the social science version of magic(how it should be working).
What Theory ignores is that belief is a very core thing for any magician to have. It is their belief that shapes the magic, and enables them to summon the spirits they summon. From what I've read, the Unified Magic Theory says anyone can summon any spirit. Arcana says that if you don't believe in it, you ain't gonna summon it. But it lets you write out the formula for somebody who does believe in it. Alchemy is actually a logical outgrowth from Enchanting using the Arcana skill to figure it out and develop it.