Why do some oscilloscopes have several input channels?
You're right - the main reason to have multiple input channels is to be able to measure several signals simultaneously. A two-channel oscilloscope lets you compare an input signal to an output signal, and if you had a means to control the behaviour DUT (device under test) in between the two probes then being able to see the input and output simultaneously can help you make better decisions on its performance.

EDIT: To give another example, you can use one channel to trigger on an activation signal (for example, a pin select) and another channel on an input or output pin. Having both on screen at once allows you to measure the delay from one event (the trigger signal) and another (the input/output signal).