As you have quite rightly stated, the bandwidth command is used to specify the bandwidth of the connection that can be used for some of the routing protocols calculations for routing path decisions.
The speed command is used to identify the ports speed so it can be manually set, its used for your Ethernet Ports so you can set your Speed and Duplex of that Ethernet Port. So, if you are connecting the Ethernet1 to a Switch Port that has 10Mbps, then you would have Speed 10 and Duplex Full.
If you were connecting to a 100Mbps Switch Port, then you would have Speed 100 and Duplex Full.
If you were connecting to a 10Mbps HUB port, the you would have Speed 10 and Duplex Half (since its a hub)
Serial connections do not utilise Speed and Duplex, therefore you cannot select a speed on this type of port.