The main benefit doesn't have anything to do with bandwidth (which is very cheap), it's that the "disable your ad block" nag screens actually work on some peopl

The main benefit doesn't have anything to do with bandwidth (which is very cheap), it's that the "disable your ad block" nag screens actually work on some people. If even a small percentage of people actually do whitelist/disable their ad blocker as a result of the message, then the site makes more money.