I can't tell you how many times I read that advice and ignored it. I figured a metronome was just a nerdy machine for piano players. I couldn't see what difference it could make, especially since I was most interested in bluegrass, a folk, improvisational music that relies on intuition rather than mechanics (or so I thought).
I finally got one, and I was astonished at what a difference it makes. When you play with a metronome, you have to stop kidding yourself and start being honest. You THINK you play in time, you think you can play a tune, you think you're doing it, but until you practice with a metronome, you're just fooling yourself. I bought a wind-up mechanical kind about 15 years ago, but there are all sorts of digital ones now, and inexpensive. If you don't have one, get one. I guarantee it will be your best musical investment in a long, long time.
Its uses are myriad, and I'm sure others can tell how they use it. Here's a list of mine:
Practice scales at speed. I like to start at half note=108 beats per minute and play scales, increasing the increments until I reach my limit. Also practice scale patterns doing the same. The benefits for both right and left hand are enormous.
Practice tremolo. There's no better way to regulate what you do in a tremolo than using a metronome. You can feel what both you right and left hands are doing with a precision that you can never chart without the machine.
Practice rhythm. This is great before playing with others, and a skill many people overlook. The metronome will not lie, and it will not get tired (as long as you wind it or replace the battery!)
Practice tunes. When I'm learning a new tune, I like to find both a target and an actual tempo. I listen to a recording and set the target tempo, then play at a speed I can actually play at. Knowing the difference shows me exactly what I need to do. It's encouraging to see myself progress notch by notch.
One of the benefits of a metronome is the way the beat will ingrain itself into your brain. You can't help but become steadier once that beat burrows itself into your synapses. I can't repeat this enough, but I truly think playing with a metronome is the single best aid you can find to making yourself a better mandolinist and musician. If you practice with one every day, you will get better and steadier.