Open up high level bot in another tab. Play their moves and copy what the bot does in response.
I have no idea why people do it, it's not even like video game cheats where it gives people a advantage but you're still in control - you're literally just copying.
I’d have to guess, but fake internet points? The ability to say “I’m ranked 5th at blah blah chess website”. Maybe it’s not even to others but just to themselves.
They don't even need to open another tab. There is at least one Google Chrome extension specifically for cheating in chess (which the creators pretty much openly admit - their YT channel features a blatant cheating tutorial video).
I've reported this extension for abuse - since it's obviously unethical, goes against the rules of the websistes it's supposed to be used on, plus the description in the Store is misleading, as it's deliberately vague and unlike their YT profile, it cautiously doesn't mention cheating at all. Obviously it hasn't been taken down though.
I have no idea why people do it, it's not even like video game cheats where it gives people a advantage but you're still in control - you're literally just copying.