Why ww2 sub domains?

I have seen on the web some domain names having prefix of ww2 or ww3 or so (ww2.somedomain.example, ww3.yourdomain.example). And these happen mostly when traveling from a page to page. What would be the reason of having such subdomains? Is there anything special about them or are they just another sub domain? I mean, are they useful in any particular context?