The vertical amplifier conditions the input signal so that it can be displayed on the CRT.
The vertical amplifier provides controls of volts per division, position, and coupling, allowing the user to obtain the desired display.
This amplifier must have a high enough bandwidth to ensure that all of the significant frequency components of the input signal reach the CRT.
The trigger is responsible for starting the display at the same point on the input signal every time the display is refreshed.
It is the stable display of a complex waveform that allows the user of an oscilloscope to make judgments about that waveform and its implications as to the operation of the device under test.