You have to consider several things. First, depending on the composer, many of the works they created were under comission or they didn’t have a specific mood in mind when creating the piece so it was easier to name the pieces by the musical form and the key it was in. Oddly enough that creates an index that’s, partially easier to go through. We could consider those pieces were never meant to be “baptized”, formally speaking.
But, sometimes, they did give a name to a piece (that’s often added at the end of the technical name or, sometimes, replaces it… although sometimes those weren’t great names XD). And, on top of that, sometimes people assigned a nickname to it (even when the composer didn’t give one). In such cases the only way to still know of what piece you are talking about is by using the technical name and the Opus number.
And, on top of that, there are the catalogue names. Since some composeres created a lot, academics later on created indexes of their works in order to find them a bit more “easily”. You may have noticed Mozart’s works have a K. and a number most of the time; that was not added by Mozart, but instead by Ludwig von Köchel, who created a catalogue that sorts Mozart’s works chronologycally (and that’s still being updated today).
That leads to complex, to the point of sillyness, names but it serves a purpose: to have a system (understood by every musician or music lover worldwide) to easily identify a piece among hundreds.
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