I don't know if this has been covered yet. Since I am only at episode 10 it could most probably have been discussed already. So this is merely a reference for myself and maybe for any other curious developer who stumbles accross this question:

What's that "u" really good for?

The answer is simple:
While Microsoft supports anonymous structs as extensions (which makes it really handy in especially this case), this is not really portable code.

So by providing a named struct ("u"), Microsoft at least allowed people writing portable code whatsoever. :)

That's it.

At least this is how I understand it. But maybe someone else can clarify this better than I can.