Raspberry Pi is good for testing the minimum resource requirements needed to run Monero. As long as Monero continues to work on ARM devices like the Raspberry Pi then scaling concerns related to hardware requirements will not be a problem for most people.
It also is extremely inexpensive in terms energy consumption, with a Raspberry Pi and other ARM systems able to operate as a full node with only 1 or 2 watts of electricity.
Security benefits of using a Raspberry Pi result from it being isolated from other irrelevant (to Monero) programs installed on every modern PC. Such an isolation model is ideal for creating offline wallets for cold storage.