The PCM plugin layer was designed to offer a simple rate and format conversion routines which behave in the same way as the PCM direct access layer. The ALSA native PCM driver does not do any conversions itself. Everything must be done in the user space.
The using of this layer is very recommended, because some common hardware cannot do some standard things like full 16-bit duplex. For example, ISA Sound Blaster cards have always one channel 16-bit and second 16-bit.
Unfortunately, the PCM mmap mode is not available in the plugin layer, because the direct access into the audio ring buffer does not allow any additional conversions.