With many programs (including MySQL), the default and sometimes only way to authenticate a user is to ask the user for a password. It works the same even if that user is actually another program: the server will still ask it for a password, and the program must know the answer.
The easiest way to ship a product with all the programs knowing the right passwords is to use a bunch of hard-coded (as in, part of the ISO image—the same for every install) ones—then you can just put them in the various config files (or worse, in the various programs' source code). An alternative is to generate unique, random passwords for each install and have the installer put them in the appropriate config files, but that's more work (though far more secure!).
A second reason for default passwords is to make writing documentation easy. "Log in at http://machine-ip-address/, user admin, password admin" is easy. (This doesn't apply to program-to-program passwords, of course).
The key thing about these passwords is that default passwords are always insecure. It does not matter if the default password is
p455w0rd, or even
j@'kNDv&178VGzsv; it is insecure. Its been published widely on the Internet, it is known to anyone who cares to do a slight bit of research, and it's probably on any password-guesser's wordlist. So it really doesn't matter security-wise what the default passwords are, and thus
passw0rd (instead of
password) is probably just a joke. The person who chose it knows its insecure (since its a default password), and makes a joke of "must contain letters and numbers" security requirements by putting a clearly insecure letters-and-numbers password.
So, yes, when technically possible, you should change default passwords. When it's not technically possible, you should ask why and think long and hard: is it really a good idea to run this software? And if you do decide to run it, make certain to control access some other way.