The Development Kit is designed for hardcore developers who would like to take their RuuviTags to the limit by flashing and debugging the devices via SWD (Serial-Wire-Debug) bus.
The DevKit isn’t needed if you’re happy to flash your RuuviTags over-the-air.
For most of the people, DFU OTA (Device Firmware Update Over-the-Air) is enough if no protocol stack or bootloader has to be replaced or no serious debugging is needed.
Note also that if you’re developing your own firmware, power consumption measurements are recommended so that possible current leaks can be obtained. The best tool for this is Nordic Semiconductor’s Power Profiler Kit which is compatible with the nRF52-DK evaluation board.
To read more about the DevShield, refer to RuuviLab’s tutorial.