For many (maybe even most) people they do the same thing. Some just want a HTPC, basic desktop, or small server, no GPIO needed. Raspberry Pis were definitely cheaper early on because you could actually buy boards at or near MSRP, but over time the price rose and mini PCs became cheaper.
or maybe the mini pcs started becoming available at all. I think these were a concept since core 2 duo and first gen i3 era. before then the chips were too much wattage for a mini pc. and the first generation mini pcs were also costly for enterprises to adopt in bulk. its only after 4th and 6th gen intel that these were adopted in large quantities in enterprises and once they dumped those in the used market they became comparable in pricing and somewhat in power to a rpi.
Used powerful PCs have always been available for dirt cheap on eBay even when the first pi was launched. just that mini pcs are a more recent thing.