“Our automated systems analyze your content (including emails) to provide you personally relevant product features, such as customized search results, tailored advertising, and spam and malware detection. This analysis occurs as the content is sent, received, and when it is stored.”
As said, they read and analyze your mail, mainly to sell it to advertizing companies. Google make money with this.
Weirdly enough, I have had less spam mail on my gmail account, than I have on the account of an e-mail provider from my country.
In my country popular free email providers send many spam messages to users by themselves, so inbox looks much worse than in gmail.
Anyway, Gmail is a no go. From Google TOS
As said, they read and analyze your mail, mainly to sell it to advertizing companies. Google make money with this.