When it comes to designing, it is for instance of interest which devices the visitors have used so far to visit the site.
Piwik already collects quite some details on this (e.g., device type, model, brand, and (physical) resolution).
However, what I just noted is that Piwik seems to display (only) the physical resolution of the visitor’s device in reports. With high-resolution screens on desktop computers and mobile devices we see today that the resolution is scaled to still produce readable content (i.e., one “CSS / visible pixel” is displayed on multiple physical pixels, see also iPhone 6 Screen Size and Mobile Design Tips - Updated for iPhone 8 & X!).
Thus, in oder to infer details about the screen real estate, it is from my perspective not only necessary to display pyhsical screen resolution but also the (smaller) available resolution. Is there a way to display these dimensions as well? For instance, the “global visitor configuration” only shows the physical resolution.
Related to this, I would think that for some uses also the actual viewport of the browser window is of interest (e.g., for the following questions: What is the typical user behavior on desktop computers / laptops? Do they open the website full screen / in smaller windows?)