It can be used in a few ways

primary reason to do it, in my mind anyway, is as a streamlining drill. Keep the bottom arm extended against your ear.

Other things: it gets you rotating completely from side to side. A little more than you would do when you just swim. And it gives you lots of time between strokes to really focus on getting the forearm vertical and engaging the big muscles in your back to help the stroke.

It "can" be for kicking, but tha's not the pest use of that drill. If you want to kick there are better ways to go about it.

I wouldn't call it a "6 beat kick" drill though. 6 beat kick is just swimming. The more common name for this drill is "6 kick switch" The 6 kicks isn't all that important, it can be 5 or 7 or 8. what's important is that you pause for a few kicks on your side before starting the next pull.

It's a really common drill. Virtually every club does it.

This also highlights the importance of knowing "why" you are doing drills and what they are supposed to be doing for you. Most drills are designed to isolate some aspect of the stroke so you can work on it. Just doing drills is pointless, unless you know what it is they are supposed to be isolating and what you should be focusing on when doing them.

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I do it all the time. Make certain you rotate to but keep your head straight - as Jason said it is a streamline drill but I prefer to call it a body position drill. Also helps with breath timing for those who need it...