Why would a waitress tell a customer that that's her daughter is working with her?

Highly context dependent. In general, I've found (some) parents like to flaunt their offspring whenever they get the chance. But it's also possible she needs something in particular, such as a match up.
And another thing, don't look at people as their jobs by default. This will earn you no favour on their part, and even a lowly waiter can spit in your coffee.