The mother sauces are starting points, or foundations for other sauces and dishes.
For example, a béchamel is a good foundation for a killer mac-n-cheese. You can also make a mushroom-peppercorn sauce for that steak starting with a béchamel. If you can do a béchamel, then you can do a roux, which opens up more options, my favorite being cajun gumbo and etouffee.
You can take a basic tomato sauce in many directions. You can go Italian, Spanish, Creole, or any number of nationalities. You can add meat to it, or your favorite vegetables to serve with pasta. You can mix it with cream and herbs for a great soup.
Those are the two mother sauces I know and use.