What is VAR knob used for and why should it be usually in CAL position in analog scopes?
When the "Var" knob is set for "CAL", then the vertical sensitivity (or other function) matches what you see on the screen.  If you have the range set for 1 volt per division, then "CAL" = 1V/div.

But the "Var" knob allows you to adjust the trace on the screen to something less than the calibrated value.  This may be useful for examining the waveform or comparing it to another waveform, etc. 

The reason for setting the "Var" knob for "CAL" is where you are using the waveform on the screen to actually MEASURE the VOLTAGE amplitude on the screen. (Or the X-axis TIMING on the screen.) If you are not MEASURING the voltage or timing on the screen, then you can set the "Var" adjustment to wherever is convenient.