It could be targeting people that already have such a website's tab open somewhere in the browser.
Assuming the user opened the website and didn't notice the redirect (this is more common in mobile), then forgot about it and when they opened their browser again a few days later, their favorite gambling website was waiting for them, and proceeded to gamble as they usually do.
Assuming the user opened the website and didn't notice the redirect (this is more common in mobile), then forgot about it and when they opened their browser again a few days later, their favorite gambling website was waiting for them, and proceeded to gamble as they usually do.