SD card interface works with FAT16 or FAT32 formatted cards. 3.3v level shifter circuitry prevents damage to your SD card

Real time clock (RTC) keeps the time going even when the for arduino is unplugged. The battery backup lasts for years

Included libraries and example code for both SD and RTC mean you can get going quickly

Prototyping area for soldering connectors, circuitry or sensors.

Onboard 3.3v regulator is both a reliable reference voltage and also reliably runs SD cards that require a lot of power to run