Apart from turning the relay on/off it offers the following features
The device is managed via a set of commands published from any MQTT client. Statuses are published by the device. Subscribe to them on your MQTT client.
SSID/Password management - The local SPIFSS file system is used to store an array of WiFI credentials.
Extra GPIO On/Off. There is an 'extra' GPIO available (next to the Ground pin on the UART breakout on the Sonoff.
https://github.com/mgaman/Sonoff-modified