ISPs can observe your DNS lookups to their servers and assemble a profile on you based on the domain names you look up, and put you into a series of audiences that marketers can then use (for a fee) for ad targeting.
ISPs can also observer your DNS lookups to Google’s or anyone else’s public DNS servers.
ISPs can snoop on your unencrypted traffic, proxy it, and inject headers into HTTP responses to facilitate (you guessed it) the creation and sale of audience data to advertisers.
ISPs can transcode (and downsample) multimedia content to decongest their pipes or airwaves.
If you are a spy or a member of a disfavored political group, you should almost appreciate the scummy practices of ISPs, as it drives a bunch of non-spies and people not associated with disfavored political groups to adopt privacy-enhancing technologies.
If I worked at the NSA or CIA or FSB or Mossad or wherever, I would highly encourage lawmakers to enact laws to protect consumer privacy in order to drastically reduce the perceived need for people not in the above groups (et alia) to adopt VPNs and other technologies; there would be fewer “boring” people using such technologies, giving the needles a lot less haystack to get lost in.