Because the hackers have that same list of 500,000,000 passwords so when they steal your password hashes and want to brute force your passwords, they can start

Because the hackers have that same list of 500,000,000 passwords so when they steal your password hashes and want to brute force your passwords, they can start with the 500 million most common passwords instead of having trying random strings (there are 208 billion random 8 letter lowercase strings)