Some years ago Google was still advertising that they never delete any search related data. So anything you'd have searched using google since the 1990s is still there.
Google cookies have had the expiry date of 2038 for a long time.
And at least years back deleting any emails permanently from gmail was a chore.
All of which raised enough questions, and made me not always want to give google or its affiliates cookies.
Well, they like to get and keep data from their users. Like with anyone or anything that has enough data of you, they know your habits. Your doctors, your credit card companies, your grocery, friends, significant others. The more data about you they have, the more they generally like you. And they (google, your doctor, your credit card companies) don't collect the data for just all-good and non-profit purposes.
Optimizing the search results for you is one application for using your data. Law enforcement (as widely as it's defined by the PATRIOT act) is another legitimate application for viewing the data about you.