It isn't that optical and coax cables can't send hi-rez, they can, but not all equipment is made to be able to send hi-res through such outputs, or there are limits to how high the resolution is.
A DAC isn't just for converting hi resolution digital into an analogue signal, they are used to convert any digital source signal into analogue so it can be amplified and sent to the speakers. That includes low-resolution, compressed files, CD quality files and high resolution files (depending on the DAC, but most DACs these days seem to handle higher resolutions. Just how high they go depends on the DAC). The amp has an oboard DAC to allow greater flexibility in the sources you attach, whether that is a TV, computer, or whatever.
As far as I can see from the manual, you should be able to output up to 24 bit 96kHz via optical, although it isn't 100% clear/explicit about what you can and can't output via optical.
See pages 12, 31 and 32 for settings.
http://www.superfi.co.uk/images/manualbrochure/PANASONIC-DMPBDT-MANUAL.PDF