Gmail s’ Last Account activity

Saw a link called "Details” in the bottom of the Gmail Page. - Last account activity     As usual Google into their Ad business in a different way.......

Clicking the Details link next to the Last account activity line at the bottom of any Gmail page shows information about recent activity in your mail.

Recent activity includes any times that your mail was accessed, using a regular web browser, through a POP client, from a mobile device, etc. We'll list the IP address from which the access was made, as well as the time and date.

Its shows the last 5 sessions

Google into their Ad business

How this is linked to their buisness?