Why do we need to have the 80% vaccination rate for the coronavirus?

The 80% percentage vaccination rate theory is based on the basic reproduction number R0.
The R0 number is how many people can be infected by one infected person.

R0: How many would one person infect other people in real world settings?
So for this particular virus, R0 is between 2.5 and 3.5.

We take the maximum R0 where one person would infect 3.5 people.
So we computed, we need 80% of the population to be vaccinated to protect all of us based on the following computation.
1 - 1/number_of_infected_people
= 1 - floor_to_one_decimal_place(1/max(R0))
= 1 - floor_to_one_decimal_place(1/3.5)
= 1 - floor_to_one_decimal_place(0.2857)
= 1 - 0.2
= 0.8
= 80% vaccination rate.