It may depend on the age of the students, but I introduce the use of the metronome at a rather early stage.
Violinists are less likely to play alone, by themselves.
Whether you are accompanied by a pianist who provides the harmony, or in a chamber music setting, or in an orchestra, you almost always play with someone else.
In order to play music together, especially with more people involved, you need to have developed accuracy as individuals. The more accuracy you have, the tighter the ensemble gets.
I have studied a lot of chamber music, especially sonatas for violin and piano. Once I had an opportunity to work with a very musical pianist. When we played together, I felt the music became so alive and very interesting, but I also noticed the music was not stable because the playing was lacking the accuracy.
There is a pulse underlying the music.
In English, pulse is a word used for a heartbeat.
In other words, the pulse is the heartbeat of music.
When a young student starts rushing while playing, I jokingly tell him: “Huh? Your music got a heart attack?” or “Your music got heart disease?” trying to touch a very important point on playing music, and introduce the idea.
Playing at a steady tempo also means that you are providing a healthy heartbeat to the music.
And a healthy heartbeat in music must be provided by the performer.
Teen students who came from other teachers sometimes play very expressively, which is wonderful, but they can’t keep the steady tempo. Ritartdando, accellando or any other tempo change can be effectively made, only if the player has the ability to play the music with a steady pulse.
The first step in using a metronome is to be able to play with it in time, but the real goal is for the performer to develop an accurate pulse within himself, to be able to keep the accurate pulse without a metronome. In other words, playing along with the metronome is not enough. It is only the beginning of a learning process.
I once realized this by myself, and I will never forget my own Aha! experience. 🙂