If you don't do any logging and don't want to know what your users are doing - it means that you won't have to deal with the cops as much. And there won't be any risk of those logs getting leaked or stolen .
Unless you're de-facto part of the government like Google and Microsoft - I see no good reason to log anything more than what's legally required.
1. Browsing habits would hardly have an affect on the vast array of data to have an effect on ads presented to you, unless you care about your privacy. Its all target auidence and marketing (look at ExpressVPN or Surfshark. They all offer privacy but never follow up)
2. Their algorithms can avoid showing you ads derived from the VPN if it detects the usage of your actual IP
The market for data is shady. They can simply sign contracts with a few reputable "market intelligence" firms. I am also very sure that another VPN firm wouldnt mind being a proxy seller the browsing data in return for some comission.
Unless you're de-facto part of the government like Google and Microsoft - I see no good reason to log anything more than what's legally required.