if you want the cynic's version, https will prevent ISPs or other nefarious operators between you on the website's hosting provider from swapping out the websit

if you want the cynic's version, https will prevent ISPs or other nefarious operators between you on the website's hosting provider from swapping out the website's ads with their own ads. google's customer is the website operator, so they want to protect that relationship. (the less cynical interpretation is that all the other non-ad content on that website is also valuable and shouldn't be modified in transit)