What is the primary function of a "key signature" in music?

The primary function of a key signature in music is to specify which notes are to be played sharp or flat throughout a piece. This serves as an essential guide for performers, setting the tonal framework of the piece and indicating the scale on which the music is based. By establishing certain accidentals at the beginning of the staff, composers can communicate the overall harmonic context in which the music should be interpreted. This allows musicians to understand which notes are altered from their natural form without needing to place accidentals throughout the score, thereby simplifying the reading and performance of the music.

Tempo changes, dynamics, and structural outlines are governed by other notational elements in music, such as tempo markings, dynamic markings, and form indicators, respectively. Therefore, the key signature specifically relates to the pitch content of the piece rather than these other aspects of musical notation.