Snape demonstrates that he was an extraordinarily skilled wizard, even in an aspect that is usually overlooked: healing magic.
After the confrontation between Harry and Draco in the bathroom, Snape finds Draco seriously injured by Sectumsempra and manages to stabilize him using a counter spell he had created himself: Vulnera Sanentur.
Interestingly, it was not simply a common healing spell. Snape had created Sectumsempra, so he also knew exactly how to reverse its effects. The book describes how he uses the spell repeatedly while performing movements with the wand, causing the wounds to close and the blood to stop flowing. This precision demonstrates a very advanced knowledge of curses and the magic necessary to counteract them.
In addition, the scene reveals something important about his talent: Snape was not just good at casting offensive spells. He was capable of creating magic, modifying it, and then developing a specific solution to neutralize it, which requires a very high level of knowledge beyond simply memorizing spells from the books. His mastery of Potions, Defense Against the Dark Arts, and Spells fits with this same ability.
That is why Vulnera Sanentur is one of the few scenes that allow us to understand why Snape was considered such an exceptional wizard. Harry discovers the offensive spell written in the book of the Muggle Prince, but only Snape truly knew the entire system behind it, including how to undo its effects when someone was injured.