Why do aircraft manufacturers produce families of aircraft?
There are several advantages, here are a few:


Design costs are lower for a family of aircraft; major parts of the aircraft are the same in the family.
A family of aircraft share the same type certificate. This means it is cheaper for the manufacturer to certify a family of aircraft.
Within a family of aircraft, pilots can operate all aircraft with no
or very little additional training required. This saves the airline a
lot of money.
Maintenance on a family of aircraft is cheaper since the majority of
parts are common to all family members. This means having less spare
parts in stock and less training for maintenance personnel.
For an airline there is often a benefit in having some capacity
diversity in a fleet. A new route often starts at low capacity. When
the volume of passengers rises replacing the flight by a  larger
aircraft from the same family is not disruptive to crew schedules.