Older OpenWrt supported routers can be used as a 100Mbps managed switch. Most of them would have 4-5 ports, including the WAN ports. With managed switch, you can configure VLAN, create isolated networks, etc. Managed switch are useful if you're dealing with access and trunk VLANs, also, you can use a single switch for different networks, which you can't do with a unmanaged switch.
IoT devices works in 2.4Ghz band, you can create a private network only for your IoT devices, with no access to your home network, and if you've multiple routers, you can trunk the VLANs over your normal cable to extend it to other OpenWrt routers.
You can also create a public network with a captive portal.
Keep in mind that most ADSL/VDSL modems doesn't support OpenWrt, in that case, you can use your device as a unmanaged switch with a wireless access point. You can wire it at different locations over your house for increase wireless connectivity.