If you don't know what's really going on with the interfaces on your Cisco routers/Switches, then you likely don't know which changes you need to make.

In my opinion, a network engineer must know the “show interface” in depth; indeed, this command is useful to obtain various interface information like a drop, duplex mismatch, error, tx/RX loads, …Usually, the IOS switch/router have a similar “show interface” output; the differences are dictated by devices, interface, and IOS.