It is important in music because the crescendos, the tone quality, and note values, all of these things work together to create a sense of emotion for the audience. Professional musicians do this really well, especially in the context of a symphony with dozens of other moving parts. It is the same thing with writing.
A phrase is a unit of musical meter that has a complete musical sense of its own, built from figures, motifs, and cells, and combining to form melodies, periods and larger sections. A phrase is a substantial musical thought, which ends with a musical punctuation called a cadence.
However, phrases can be any length. An analogy would be a short declarative sentence - "Stop!" "Come here." Musical phrases can be as short. If there are lyrics, look for sentence dividing or ending punctuation such as commas, semi-colons, colons, periods, exclamation or questions marks. Try singing the melody line.
