The point of vaccines in general is to give people some ammount of a weapon to ward off health issues. Usually people get sick anyways but the point is to lower the severity of whatever it is so you're not "out of the game" so to speak for a week or more; and can get back to work asap.

If your chickens get sick and it starts to spread and affects the whole flock then your egg production goes down and if you rely on selling eggs for money then you start to lose money. We are the chicken.