Band practice comes, and you notice the band seems to speed up and slow down during songs, or not get the right vibe from a song because the tempo isn't right like it was last week. What do you do? Stop and start again? Grind your way through to the end of the song? Blame the cosmos due the planetary alignment? Put the song back on the shelf and try again next week?
Try writing and rehearsing with a metronome.
Unless your drummer has a metronome up his ass (and I've met a few that did), you miss out on establishing and fine tuning your sense of timing. With a strong sense of timing you can have a huge impact on the power of your rehearsals and therefor the tightness & solidity of your band. The most noticeable benefit will be when you sit down to record your songs or head off to a studio and find that your guitar takes sit perfectly together or the backing vocals are really tight. You won't have to grumble at your drummer for speeding up or slowing down during drum fills and transitions (even though he's probably got a click track in his ears). If the drummer hasn't rehearsed to a metronome or performed the song at a consistent bpm, then how can you expect him to deliver flawless takes? "Oh, but we want it to waver in and out of time, makes it organic man" - Bullshit. If you could play tightly together, you would.
Of course there's quantization and various methods of "Flexing or Flex Editing" but that's not the point. If your producer or engineer has to deal with crappy timing, you'll either need to fix it yourself, pay them for it or settle for your crappy timing in the recordings. The point is using a metronome to give you a base of power to control your performance. Even the softest most dynamic song can be full of power which brings me to my next point.
Competently playing around the beat (using the example of a vocal for a soft moody song) is made possible by knowing where the beat is! So, to play out of time well, it makes sense to know where the time is.Tight syncopation can only be achieved after hours of programming yourself to play in time.
There seems to be two types of musicians in relation to timing. Those who have "Learned" rhythm (had to work harder than most to develop average rhythmic consistency) and those who have "Natural" rhythm (rhythm in the blood or bones). If you can get from the start to the end of your tune without falling off the metronome then congrats, you're probably awesome to jam & record with. If you think you're solid but have never practiced with a metronome, then you're probably not.
Either way all of us benefit from practicing with (and without) a metronome.
