I think metronomes help every guitarist. If you've never used a metronome and you just play by what you think is rhythmly correct, you don't notice it yourself but you usually have sloppy rhythm. Naturally people don't have a great sense of time, or at least not precisely enough.
Test yourself by looking at a clock and when it starts a new minute, count the first 3 seconds, then look away from the clock and keep counting, when you think the minute is up, look up at the clock. If you have precise rhythm you should be dead on. You'll probably get fairly close, maybe by one or two seconds but in metronome time it 60 per second. That's alot to be off, when your thinking in sense of music time. If you have a stop watch, that's even better.
What I'm trying to say is a metronome helps with your sense of time, or rhythm in music sense. The better your rhythm, the better guitarist you are. Those who don't use it, try it for a couple of weeks and watch how your playing will improve that much. A metronome has many benefits:
It gets you to pay attention to the rhythm your playing. It increases speed. It's also gives you a better sense of rhythm notation and time.
Really test your self by using extremely slow metronome timings like 30-50 bpm. This will really improve anyone's rhythm, cause you have to be more precise. Also try timings that don't naturally fit the part your playing. Say a part is 90 bpm, try it at 110 or 70 (where it seems unnatural). Both are great workouts for even a experienced metronome player.
hope this helps.
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