"You" in this case are a hacker / social engineer trying to break into someone's account. If your target puts gibberish as the answer to that question, here's how the conversation will go:

Operator: What is your mother's maiden name?

Hacker: Smith (let's say this is the real name, gleaned from public records)

Operator: That's not what I have here

Hacker: Oh, you know what, I think I just put gibberish when I signed up. I thought that would add some extra security, but I forgot what I wrote, ha ha, joke's on me. The real name is Smith though.

Operator: Story checks out, I'm giving you access to the account now.