Yes, extras are very very important to build the narrative in a scene, and a genius director can make the character speak through the extras, while the main actor in focus can remain silent.
Take a look at this picture from The Shawshank Redemption (1994) – IMDb,a perfect and mind-blowing use of extras.
Brooks just got out of prison after many many years. He is feeling so lonely and terrified. The emotion gets pumped to our subconscious as all the extras in the bus look away from Brooks, they all look sideways, making him look even lonelier than usual.
A good director uses the extras to provoke some emotion inside us, and not just use them to make the scene be more realistic and beautiful.
Mind=Blown
It's a beautiful life 