Spatial References I can understand - when you want to create a map of a single country/region, a custom projection centered specifically around that region can provide a more accurate approximation of equal-distance points, parallel lines etc, something that is not possible with a worldwide projection.
Note that Redis Geo uses the infamous Web Mercator, used also by Microsoft Maps, Google Maps, OpenStreeMap, MapQuest, etc. but apparently rejected by the standards body EPSG with the declaration: "We will not devalue the EPSG dataset by including such inappropriate geodesy and cartography." Ouch. [1]