There is only one router on your network. It routes traffic from one machine to another. This is typically also the gateway, and it only has so many ports.

If you want more physical devices connected to your network, you’d need switches to fan out your network.

Un-managed switches essentially takes packets from one port and pass them through another port, easy peasy, nothing fancy.

Managed switches, however, can do more than just take packet from one port, then push it out to the other side. You can set up link aggregation for example, allowing more throughput by using two or more ports to go to the same destination (maybe for example a central file server). You can have L2 vs L3 switches so they route differently. You can have multiple paths to reach another machine, for redundancy but must implement STP to prevent broadcast loops etc.

Once your network grows larger than just Internet for a couple of desktops, it gets a lot more interesting.

Certainly not an expert here but the GUI “being there” means you can configure something about the traffic flowing through, maybe VLANs or QoS. That also might be why some switches have fans. Deciding what packet has priority or is allowed is a bit more computationally complex (read: heat generating) than just pushing a packet to the right address.

You might want a VLAN if you have a server connected to the same switch as your PC, but they shouldn’t “see” each other. If you didn’t have a VLAN there, your router or firewall can’t manage anything about the connection. Say you have a website and database on your server and only the website should be accessible by your computer, you’d be able to configure that with the firewall.

A managed switch allows you to have vlans, routing, QoS, spanning tree protection etc. You don’t necessarily need a gui, a lot of them are cli only, which is preferable but less user friendly if you’re not used to it. Depending on your needs a managed switch can be overkill.