why is it important to include the key in the name of composition (e.g., Symphony #_ in __ minor)?

Answer 1

It's a tradition and NOT necessary. For example, one might simply state Symphony number 104, The London Symphony, by Haydn. The Key adds a bit of info, that's all.

Answer 2

It's just a way of identifying the piece. Sometimes a piece is referred to casually by its key only, if it is clear which piece is meant. It is deceptive actually for a symphonic piece, which may start out in the title key but will soon move to another key, and travel through many before returning to the title (home) key. What's more, the later movements are not likely to be in the same key. So the name of the key in the title is just a convenient way to identify the piece.

Answer 3

It is an important identifier. For example, when someone says to me "I am doing the Bach Partita #5, I have to search my memory, go through the key patterns of the first four, and then I arrive at which one they mean. If they say the G major, I know right away which they are talking about.

In Haydn's piano sonatas, the cataloging is so screwy, that you have to include the key.

It really nails down what it is that you mean to say!

Answer 4

Because if we know the key we know that how should we put our fingers on the instrument.
For example if someone talk to me about a piece of note in G Major so I can think and after that I can understand how is the note or what is the note.

Answer 5

Because there are different parts of a symphony that sound different with different tones. The wrong key completely changes the sound of a symphony as does having the wrong portion.