An account checker bot is an attack tool that takes lists of leaked username and password pairs and tests them against a website.
The attacker uses account checker bots to test stolen credentials. The attacker may then also use this account for other malicious purposes such as spam. Successful logins can result in account takeovers.
Attackers can find and use publicly available lists of credentials, in combination with account checkers, to find vulnerable accounts which they then use for various fraudulent activities.
Many websites cannot distinguish bot interactions from normal human interactions and consequently accept this type of bot traffic.