If you run a restaurant, and you serve soft drinks, you can serve Coca-Cola Products or Pepsi products.
Many years ago (before 1997) some restaurant chains objected to Pepsi products because Pepsi owned restaurant chains including Pizza Hut and KFC, and cross promoted its drinks with the restaurants.
Back then Pepsi would pay restaurants to use their products in stead of Coke. So they were able to overcome some of the competitive objections to using their products. Coke never paid.
In the late 90s, Pepsi solved the problem by a corporate separation of the restaurants and the drinks. The restaurant company is now called Yum! Brands. I assume they stopped paying restaurants to take their products.
To me Google vs Yahoo resembles the Coke vs. Pepsi situation. And, it is just as important.