Retaining Terminated users in AD

I'm taking over some responsibility managing users in our AD. Our old sysadmin had a "Disabled Users" OU for all former employees. It's never been emptied, and has 250+ entries. No one in my organization can answer why we have this. Do you guys retain the accounts for employees you let go, and if so, for how long? I believe our Exchange retention policy keeps their mailbox for 30 days after their account is deleted.