Spoof screen size
Makes browser fingerprinting more difficult. The Tor browser does this by sending screen sizes in buckets (phone, tablet, desktop).
Updated by Soren Stoutner about 2 years ago
Also, it should be noted that websites that require screen size spoofing to display correctly should be considered to be buggy, because if they were written with correct responsive design (webpages that work well at any resolution) instead of the old-fashioned adaptive design (different versions of websites for different devices) it wouldn't be a problem.