Question: ![The oscilloscope I will be using](https://i.stack.imgur.com/qjAq6.jpg)

At my university, I'll be using the ELENCO S-1325 oscilloscope for a instrumentation lab. Since I'm a mechanical engineering student and not an electrical engineering one, I don't know exactly how to use all the functions though I am familiar with the basic use and operation of it. However, I want to know how to use some of the more complex functions of the oscilloscope.

On an oscilloscope, why would anyone move the variable knobs off their “calibrated” positions? the variable knobs (red VAR knob) for voltage measurements? the small gray “var sweep” knob for the time base?

Isn't it a good thing to keep things calibrated rather than use uncalibrated settings?

Answer: 
If you want to measure the rise or fall time of a digital signal, you use the variable knob for voltage to adjust the amplitude, so that it fits between the horizontal dotted lines (5 divisions). Then you measure the time from 10% to 90% (the middle 4 divisions). Conveniently, there are even percent marks on the screen.

It's less common to use the horizontal variable adjust, maybe it could be used for measuring duty cycle.
There are many reasons why you might want to leave the calibrated setting : StarBlue touched on one, here's another.

Imagine you need to do a quick check on the frequency response of an amplifier with a gain of 3. That's usually defined at the -3dB point (amplitude=0.7) It's much easier to see differences between input and output signals if they look the same in the first place, so set up the signal generator on Channel 1 so that the input channel covers 10 of those calibration marks (on the crosshair, i.e. 2 divisions). Now set up the Channel 2 on the output ... the oscilloscope gain is fixed, and none of the calibration settings give you exactly 2 divisions, so use the next largest one and de-calibrate the gain until you have 10 calibration marks.

Now you can adjust the input frequency and observe the frequency response. You need the frequencies where Ch1 covers 10 divisions and Ch2 only covers 7. If the signal generator amplitude changes with frequency, you will see it on Ch1, and adjust the sig gen to compensate.