On screens high-resolution tiles have the effect of making everything in the image look exactly the same but bigger (it might feel like you're seeing less detail). When printing, this means you get more dots per inch, sharper print quality, but without scaling down things like text labels to become too small.
The scaled-up features of high-resolution tiles (text in bigger fonts, larger icons) will help with visual impairment.
Slippy maps may be configured to show such tiles in a scaled down size by default and the user's browser can enlarge them again if it is set to enlarged web pages (including images). Or maybe the slippy map software could read the user's dpi setting and scale accordingly (just an idea …). This may be useful on high-resolution displays to achieve resolution independence for slippy maps.
- retina tiles by lyrk
- RRZE test service presenting double resolution maps (but scaled to normal size in this view. This is a "retina" presentation which will look good on a mobile).
- The idea is also illustrated nicely here in relation to the geofrabrik WMS service (paid for).
- BBBike map compare with retina maps
- a highres tile map by OsmAnd
- Mapbox APIs for map tiles and static map images have an option to return Retina (2×) images.