The reason that tab books have the bpm is for people that have never heard the song before. After hearing the song, you can easily approximate the speed with aural memory, but how can someone remember perfectly how fast the song is if they'd never heard it? They'd crank up their metronome to 112 bpm and jam away. After this initial hearing (listening to themselves play it), they'd get a good idea for how fast the song is supposed to be and probably not need it again.

You can also view the metronome as "weight training" for the guitar. I mean, in all reality, a bodybuilder could just go lift heavy stuff like logs, people, cars, whatever he could get his hands on, but in the gym he can really see his progress by having done 10 reps bench pressing 150 pounds last week, and 10 reps at 155 pounds this week. The metronome is like this weight machine. A guitarist can practice speeding up just tapping their foot to keep time, but with the metronome they can see their results - completing a chromatic exercise 10 times perfectly at sixteenths at 110 bpm this week, and 10 perfect "reps" at 115 next week. Being able to really see your development is quite helpful in staying inspired. People that are on diets weigh themselves often not just to make sure that their diet works, but they also feel good when they see they've lost 5 pounds... bodybuilders measure their biceps, chest, waist, etc... guitarist measure their speed with a metronome... it's all about building self-confidence and seeing that your practice has a purpose (and being able to say, "Dude, I can bench press 400 pounds now!!" or "Dude, I can shred 16ths at 160 bpm now!!")