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You may need such cap to make your opamp stable. Amplifiers that are made of more than one stage can get unstable. Almost always you need to make the opamp stable for the entire frequency band and the most common way to do that is by using a comp cap. You can use it in a miller connection for instance.
I think you should read a little some basic book, like Razzavi. You could also just try at the wikipedia or do a search in this same forum.
Well.. In a two stage OTA topology, for miller compensation, we introduce a capacitor between two stages.. That capacitor moves the dominant pole to lesser frequency and second dominant pole to higher frequency! By this we get a higher unity gain frequency! But the compensating capacitor also introduces a right half plane zero! As you might be aware, a right half plane zero wil decrease your phase shift much like a left half plane zero!
Hence by introducing a resistor in series with the capacitor and by appropriately choosing the value of the resistor, we can move the zero to left half plane which contributes in increasing our phase! Thus along with the frequency, our phase also increases!