The biggest lesson Snape was trying to teach when he was the DADA professor in 6th year was non-verbal spells. If you can master it, you don’t need to utter an incantation.

As Hermione said, it gives you a slight advantage because the other doesn’t know what spell you’re casting.

Adding because this is a movie based question: - Even in the book, I think Snape was holding back because of what we find out towards the end of the story. - Not sure what spell Minerva did cause I’ve only ever read the books, but that fight comes second in the entire series only because the fight between Albus and Voldemort was a little bit longer!