USB and I2S inputs will give you the ultra HI-RES options like DSD512 and PCM 384KHz not available in other inputs, usually RCA coax would give up to 24/192KHz (which is very decent by the way) but if you want to try even higher you need USB or I2S. Of course you need to find music in these formats, not available through streaming.
I am buying these formats from NativeDSD and for streaming I prefer TIDAL