If they can charge for it, why not? Search is an important part of mail for many users, so there is some cross-over in knowledge/experience that can be leveraged.

And assuming you mean Google, the question is based on a false premise. They were an advertising company years before gmail was introduced, and an ad-tech company being able to trawl your mail to find ways to advertise at you is a very different proposition to a search company releasing the same product.

To provide personalized ads?