A 4G/5G router creates a "real" WiFi network rather than a personal network.

Hotspotting is great for 1 or 2 devices, but devices are reluctant to connect to a personal hotspot automatically (Apple devices, at least). You have to manually select/tap the hotspot every time, which gets to be a pain when you have multiple devices and you're using it as your daily, primary internet connection.

I might also want to leave the house/hotel room/office for whatever reason with my phone but keep other devices connected. In this case the router can either be fixed in place (and plugged in) or travel with me on battery. Flexibility!