Compressor pedals are made to connect between a guitar or bass and an amp, so they are instrument-level devices, as opposed to rack-mount compressors, which are designed for the studio, or to be used in an effects loop, and are line-level devices. Typically, rackmount compressors contain higher quality components and thus offer superior sound. They also provide more parameter control than stompbox compressors. That being said, stompbox compressors don't need the quality nor the amount of control of rackmount compressors, and thus are correctly designed for the role they serve.