Why might member of congress attach a rider to a bill

A member of Congress might attach a rider to a bill in order to get a controversial provision in the bill get passed with the passage of the bill.

Explanation: A rider is nothing but another provision added to a bill that may otherwise be considered as a controversial bill in whole without the inclusion of the rider. The rider acts as a puller to the other controversial provisions of the bill that the member of Congress intends to get passed.