HQ Player is an audio digital signal processing program. It was created to replace the limited computing capacity in DACs by performing upsampling externally and feeding the DAC with the highest resolution signal it can handle. It has a wide variety of filters and modulators, including some settings that exceed the capacity of most modern computers.
HQ Player is certainly not necessary. Roon will upsample and convert to DSD very well. Will HQ Player be better for you ? Well that is why it has a trial version. Give it a go if you are curious and see what you think.