How are the Nielsen ratings and the profit motive related?

They measure the number of viewers for individual programs.

The purpose for knowing the number of viewers of each program is that the price of advertising on each show is linked to the volume of viewers. Since programing is constantly changing, and the interest of people in each program fluctuates, Nielsen measures the size of the audiences on a continual basis. The more viewers that a program has, the higher the rates they can charge for advertisements on their programs.