Many people use the same password for all of their accounts. Why? Because it is easy to remember just one password across all accounts.
But is that a good idea? Nope. If that password were to fall into the wrong hands, it potentially could be used more pervasively to the disadvantage of the true password holder.
And this is not a hypothetical concern. Indeed, recent press reports are rife with disclosures of major password hacks/leaks.