Let's take the case of the Airbus A320 family of aircraft. Airbus manufactures A318, A319, A320 and A321 planes, which all belong to the same family.
What is the need to manufacture multiple aircraft from the same family having different ranges and capacity, when one could just invest in making just two or three different planes from three different families providing a wide range of range (distance) and capacity?
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.