Upsampling is one feature a lot of people like but I use it for even more.
For me headphones setup my DAC (RME ADI-2 AKM edition) allows conversion of DSD directly to analogue, so I use HQPlayer to do ALL the DSP before the DAC. The DAC’s internal DSP gets bypassed entirely. Holo Audio and T+A DAC owners do similar. HQPlayer also manages the headphones EQ for my various headphones.
For my speakers setup, in addition to upsampling, HQPlayer handles the DSP crossover for my active speakers and room correction DSP also.
And all the above is done not just with Roon as the source but also Apple Music Hi-Res lossless, Soundcloud, Spotify, Deezer.
It’s powerful stuff !
Some people like @Chunhao_Lee even pass Dolby Atmos from Apple Music (or whatever multichannel source) through HQPlayer! And do personalised head-related transfer function (HRTF) advanced headphones EQ.
Some of the things mentioned above can’t be done by Roon alone.