Casting is the physical ritual that helps align a mage’s mind to their goal - gets them into Harmony and helps them bring the exact sorcery-effect they’re looking to create into the world. Casting is closely related to the tradition and cultural heritage of the Minor Arcana (Major and Minor Arcana) you’re performing. This is why you can inherit or learn enchantments from your Magus as a Neophyte; because those before you have already constructed ways for you to cast and preform magic. Example: throwing oracle bones, dancing, drawing up seals with chalk, making a potion with certain ingredients, engraving runes-- these are tried and true methods that create reliable outcomes.
Focus and Casting are mental exercise taken physical form. It's not about the fact that the seal is actually magical, or that the ingredients hold any mystical aspect - it's that you think they do; and when you think it, you guide magic into it. And so it follows, that most mages aren't aware of what they're actually doing; because if they were, it would no longer work.