How the Latin Ablative Case Became So Busy
Latin's grandmother tongue, Proto-Indo-European, possessed several cases, including an ablative case, an instrumental case, and a locative case. Each of these three cases had its own use.
The basic idea behind the original Proto-Indo-European “ablative case” was to show motion away from something. In this sense, you could think of it as the opposite of the accusative case. For example, if you went straight from the restaurant to the movies, “movies” would be in the accusative case because that's where you were heading. (Motion toward, right?) Meanwhile a Proto-Indo-European speaker would put “restaurant” in the ablative case because that's the place you went away from. In other words, it showed the source of your motion. So in Proto-Indo-European, you go toward the accusative and away from the ablative.
The Proto-Indo-European instrumental case went neither toward nor away from. Instead, it went right alongside. It showed something that was there at the beginning of an action and was there at the end. If you went with Allison straight from the restaurant to the movies, “movies” would be in the accusative, “restaurant” would be in the ablative, and “Allison” would be in the instrumental case because she came right along with you — she was there at the restaurant where you started, she was there at the movies where you were going.
The Proto-Indo-European locative case showed location. To continue our example, if you went with Allison straight from the restaurant to the movies on Maple Road, “Maple Road” would be in the locative case because that where the movies were.
This voyage into historical linguistics will help you cope with a case that has so many different uses. You will find that beyond the basic ideas of accompaniment, motion away, and place where, each specific use of the ablative case (and there are many!) has a formal name; for example, Ablative of Manner, Ablative of Means, Ablative of Accompaniment, et cetera. It is important that you learn the formal names of these uses. It helps to keep them straight.
In addition to distinct forms, the Proto-Indo-European ablative, instrumental, and locative cases each had distinct endings. Over time, Proto-Indo-European grew and changed into Classical Latin — which is what you are learning. Confusion came when all those distinct endings blurred into one. You couldn't tell which idea was intended: Source? Accompaniment? Location?
In the end, what used to be three separate sets of endings with three separate ideas became one set of endings with three separate ideas. Rather than give this new triple-duty case an original name, scholars lumped them all together under the name “ablative.”