What exactly would someone do if they DID randomly choose my screen name and perchance matched it up with my password....on this site?
Clearly I wouldn't want that to happen at my banking site, but a car forum?
There are a number of scenarios. Probably the greatest risk falls to those that use the same or easy to crack passwords for more than one site. For example, besides any data a person may have divulged on this forum about themselves, if the site password is the same as the password for the user's email account left on this site, then that account's data may be vulnerable. If that email account contains info on other account names such as social media, shopping, banking, insurance, medical, etc., and the passwords are the same or easy to crack (e.g. dictionary attack), it's pretty easy to create havoc.
It's also a matter of combining data from different sources. In today's environment, if there is a possible monetary gain, it's worth the effort to search. As @Racenut
posted earlier, there are plenty of places to get data, including: https://www.leakedsource.com/main/
. Note that data mining can be done at this site by username, email, ip address, first and last name, and phone number.