They inoculate people with a small enough dosage to trigger an immunity response but not an infection.
This serves two purposes, it helps build your immune system up incase you do catch it, then you can be prepared physically and then also it reduces the nodes of transmission between people. Reducing the distance that the virus can spread.
If vaccines were more widely available and used AND people had a safety net that allowed them to stay at home when sick rather than go to work/school/whatever ... a virus like the flu would likely flare up badly every other year rather than get worse every single year.