The thing about a metronome is you can't blame it for speeding up or slowing down. If you play with others, you may think, "Someone keeps speeding this up, who is it?" or, "Why do they all keep dragging?", but if you try things on your own with a metronome, you may find you're the one who always rushes that third line or holds a certain note too long.
And if you don't discover any flaws in your timing, you have the satisfaction of knowing that when things get out of joint, it's NOT YOUR FAULT!
Marje