An example from the real world -- Comcast, a large ISP in the USA, has been caught injecting JavaScript into websites: https://thenextweb.com/insights/2017/12/11/comcast-continues... It's not hard to imagine a more malicious use, like tracking or injecting adverts the ISP wants you to see on webpages.
This is only possible because the connection isn't encrypted.
Another example -- Verizon were injecting a header called X-UIDH which had a unique identifier, acting as a super-cookie that was present on all websites and couldn't be removed: https://www.eff.org/deeplinks/2014/11/verizon-x-uidh
This is only possible because the connection isn't encrypted.
Every website needs SSL.