Mozilla needs money to support the development of Firefox (and the payroll of its high-salary executives).

For now, they mainly rely on Google for that money. Google pays them to avoid antitrust cases, to show the courts that they are not a monopoly and that "alternatives" exist. For example, the DOJ once proposed that Google be forced to sell off Chrome.

However, if another entity has control over your budget, they also have control over your product. If Firefox becomes "too good" to be a true competitor in the consumer space, the funding might be reduced or even cut off.

Creating a new source of revenue allows Mozilla to improve Firefox even beyond the point Google feels "comfortable" with.