Ad rewards rely on a bot that sits afk in the lobby and records all messages that start with [AD], it then takes the string right before and right after that and sends it to a disc channel, after that, servers with that disc bot installed can check if their server's name is in the list and run a command targetting the string right before [AD], which is the player's name, this command then rewards the player.
This is basically like having an army of bots that advertise for you for free and without ties to your server.
However, ad rewards have their downsides:
-Alts can advertise extremely quickly and funnel rewards for their main account, potentially breaking your server's economy
-You can advertise a completely different server: "/ad (server1) [!]JOIN NOW /join (server2) FREE OPERATOR JOIN NOW /join (server2) [!]", all that matters is that you put in their server name after the ad command, the ad message itself is completely irrelevant so you can advertise your own server while gaining rewards at the same time.
-Advertisements aren't always a good thing, for example if I advertise your server as being free op, then players will join your server and spam you to give them op, effectively reversing the game, now the bots work for me and I get my ad rewards anyways.
Lastly, why is this even allowed? If I'm not allowed to use bots to push my server up to the front page permanently or advertise it every 8 minutes, then other servers should not be allowed to use bots to help out theirs just because they're doing it in a different way