Why should I use SerialMagic Keys instead of only HID devices?

Well, for one, that would limit you to only HID devices. Many Bluetooth devices aren't supported on iOS. Even among the devices that are supported, many of them still require application-level support specific to that device. SerialMagic Keys drastically simplifies the process of using data from Bluetooth-enabled serial devices with native iOS applications and web applications accessed from iOS web browsers.