I have some guesses:

They can artificially inflate the number of users who use their AI by including everyone who uses their search engine.

It trains people to be increasingly dependent on whatever google tells them instead of checking and reading multiple sources, and verifying information for themselves.

They hope to let people pay them to manipulate what their AI returns for certain searches (either mentioning certain things/products or suppressing mention of them).

Google search is getting increasingly useless as the unending flood of ad-filled AI slop websites dominates the search results and people will abandon searching with Google if they can't find some way to provide people with the information they are looking for. (it's not going well so far https://arstechnica.com/google/2026/04/analysis-finds-google...)