they do A LOT more than just read your email lol.
they most likely have vast databases of all kinds of information on all their users and even those who are not their users.
According to Statista, 70.9% of Google's revenue comes from Advertising Revenue. Nothing is free. Not even gmail
Pretty much, they at minimum know everything about what you like and don't like, and most likely they know one or more of your deepest secrets. But don't worry, all that info is probably just stored in a database somewhere with some type of AI or computer analytics and most likely no human has ever even seen it (yet).
Today, information is money.