Mapbox Esri Connect supports Retina (@2x) tiles by default in ArcGIS Online. High resolution Retina displays can turn non-Retina enabled maps into a rather fuzzy experience. The image below attempts to highlight the difference Retina tiles make on an iPhone 6 screen, though the clarity is even more dramatic when viewed on the original display.
Many designers not accustomed to Retina displays do not realize how poor the experience can be for a segment of their users. Fortunately, with the functionality brought by Mapbox Esri Connect, making the switch takes just a few clicks.