You can sometimes get away with software switching, like when almost all of your traffic goes to WAN anyways.

Software switching doesn't scale well with smaller packet sizes and sometimes has problems maintaining line rate speeds. In my experience network shares like SMB and broadcast traffic like mDNS (like for printers) are the first to cause problems.

Most of the time risking it / not knowing if a problem is caused by software switching is not worth the money saved.

Using a dedicated switch with lots of LAN to LAN traffic also means the CPU of the router isn't busy software switching and can focus on routing / firewalling WAN traffic reaching higher speeds and less latency.