I use a PC in a closet, streaming Roon over Wifi to a streamer right next to my DAC.
A dedicated streamer is usually more convenient to use from your listening seat, in your audio setup. But if you're talking about downloading instead of just streaming, then you need a storage location somewhere which usually means a computer, plus maybe network-attached storage (NAS).
There are also arguments to be made about a streamer's digital signal fidelity versus that of a computer (especially the digital output of a cheap soundcard).
Up to a certain price point there’s no difference. That price point being dependent on one’s system, and one’s ears. I started off streaming with a MacBook Pro running Audirvana, using USB out to the internal DAC on my Anthem STR integrated amp. I briefly tried a Bluesound Node 2i, but returned it as I could not discern enough difference in streaming quality and much preferred the Anthems DAC to that of the Node (MQA not withstanding). I’ve since upgraded to a Bel Canto streamer feeding a W4S DAC. This has given me a big increase in SQ, but at a sizable investment.
My impressions of the sound quality from a Pro-ject Stream Box Ultra S2 vs a PC laptop or an iPhone running off of its battery only through three different DACs: Audioquest Dragonfly Red, Rega DAC, Denafrips Ares II:
The streamer made all three of these DACs sound better. Key improvements were a broader soundstage, more precise placement of instruments/voices, better bass definition, and a better sense of musical flow. Not sure if the difference will be the same from an iMac vs my PC laptop. Also, adding a jitter reducer such as an Audioquest Jitterbug can be worthwhile as well for more of the same benefits as a streamer provides.