Amusician would use a metronome in practice to improve their performance of which element of music?

Rythm and inner tempo.

Explanation: A metronome is an apparatus that makes an equidistant sound, which is used by musicians to keep the tempo when they are playing. Where each sound of the metronome can be used as one "beat" of the piece that they are playing.  

So the metronome is used to improve the rhythm of the musician, and also there are a lot of musicians that practice with a metronome with the objective to develop an "inner tempo", this means to reach the point where they do not need to use a metronome to keep the beat because they can do it for themself.