Ron the Drummer and I have taken our metronome out of mothballs. Our goal is not only to tighten up the rhythm, but also nail down the optimal tempos for our newer songs.
This is harder than you may think.
Just about every song we do has more than one optimal tempo. For instance, the best speed for the choruses might be juuust a little different than that of the verses. Ditto the instrumental bits vs. the parts with vocals.
Why don't we simply play the choruses slower than the rest of the song, or the instrumental parts faster? Because the inconsistency sounds sloppy. Tempo changes generally only work if they're deliberate.
So, we decide where in the song the tempo is most important—often the chorus—and make that the tempo for the entire song. Then we work with the unoptimized sections to make them sound better at the new speed.
The funny thing is that when we play to the metronome, Ron and I always find that we're slowing down and speeding up in the same places. I'm not sure if this means that the optimal tempos are truly inherent to the songs, or if we're just on the same wavelength.