A necessary step in the activation of magecraft.
While utilizing an established school of magecraft, one must strictly follow a set protocol. Spellcasting is one step in this protocol.
An analogy would be the application process of a document, which would include application, acceptance, review, and finally issuance. Spellcasting would be the “application” step.
In the case of utilizing magecraft with a broad Foundation, spellcasting is mostly a convention. However, for those who practice their own style of magecraft, spellcasting has potent self-suggestive powers.
In the bodies of magi, the Magic Circuits which shape magecraft already exist.
Incantations are the “personal adage” that help the magi transform themselves. They are a method to efficiently activate and mobilize the Magic Circuits.
Spellcasting is not a method to speak to the World, but a method to speak to oneself. Even for the same spell, different magi will utilize different incantations due to their distinct natures.
As an aside, spells that speak to the World rather than oneself are classified as Grand Spells or Grand Rituals, and it is impossible for an individual to use such spells.

Garden of sinners Pamphlet: Kara no Kyoukai Settings Glossary