Again I have to show my ignorance and ask a newbie level question.

When I see the word "loops", my brain says "A prerecorded sequence that can be chained together into a number of measures. Normally I hear them applied to 4 measures of some drum beat that is used as a bed for other stuff to be layered on." If that is an accurate definition, that brings up a question as to how useful they are. Unless that canned "loop" is the measures of notes in the exact phrasing that I want played, how do these have value? If I need a horn part that I am writing, it is highly unlikely that there is a prerecorded loop somewhere with the horn part I need.

So the question is really "What are loops?", if my previous perception of them is incorrect?