Don't make multiple posts on the same subject.
OK, a few practical applications.
Comparator.
Say you have an analogue signal which represents the level of liquid in a tank, when the tank is empty the signal is 1volt, when the tank is full the signal is 5volts.
Now, if you want an alarm to warn when the tank is less than 10% full, you may use a comparator to give an output when the signal is less than 1.4volts.
Schmitt trigger.
Our tank level alerm is OK, but when there are ripples on the liquid surface, ie the level is varying slightly, the alarm will go on-off-on-off-on-off...etc as the level varies around 10%.
To cure this we put a bit of positive feedback on the comparator to give it some hysteresis and in effect create a Schmitt trigger.
Zero Crossing detector.
Usually used to detect when AC voltage waveforms are at zero volts.
If we are switching an AC supply using thyristors etc, to minimise switching spikes and RF interference, it is best to switch them when the AC supply is at zerovolts, which it is every half cycle. Use the zero crossing detector to trigger the thyristor.
JimB
PS, did I mention:
Don't make multiple posts on the same subject.