To get through the interview process. Their interviews are a bunch of algorithm questions, and data structures is the starting point.

When he says "data structures" he actually means "interview questions".

Data structures and algorithms interview questions are the only thing that Google and FB care about.

I can imagine an UI designer coming in for an interview. ...yeah I want you to invert a binary tree.

You joke about that, but I had a friend who interviewed for a developer Evangelist role at Google, and literally the only stuff that they were asked was algorithms questions.

Yeah, it was a 50% technical role, but Google didn't care about the OTHER 50% of the job at all...

you'll need them for the interview :P