I tend to leave my Snark tuner on my violin to check for intonation. I don’t look at it all the time, but if I’m sounding something off, I can check quickly. Also, if I’m in a situation where I can’t hear myself well (playing in a large group, or even on stage with the rest of the band) it helps.
I do absolutely agree though, that using a metronome is very important. There are many apps for metronomes, or even the Snark models are reasonable and you can even plug some of them into an amplifier if necessary.
It is very difficult to play with other people if you can’t keep a steady beat and you will drive the dancers crazy if the beat is off!