is tire browning protective, or is it already an expendable layer?

I read somewhere that a tire makes a protective layer against ozone, which causes browning.

is the brown layer actually protective? or is it something you should wash off ?
I know that the tire will make more brown stuff later, after you wash it off...

I always see people on detailing forums scrubbing off the brown stuff.

If it's actually protective, then I'd rather leave it brown. So what's the deal ?