The Ethernet FeatherWing uses SPI, as well as D3, D4, and D5. In order to probe for the presence of the Wiznet W5500 on SPI, D4 and D5 need to be set to output and toggled.
This would obviously be bad if you had other, non-Ethernet hardware connected to those GPIO pins. The default defaults to disabled and the detection is not done and those pins are not touched.
If you use that enableFeature command, or check the Use with Ethernet checkbox during mobile app setup, the feature flag is enabled in configuration flash and the Ethernet will be probed for at startup until you turn it off.