Re: Boosting amps in metal
The purpose is mainly to cut flubby-extreme low end frequencies, and a little bit of extra compression (useful for palm mutes and it helps balance the fundamental and harmonic overtones). So before the signal even hits the preamp, there's decreased headroom and those cut frequencies will not be amplified (the preamp can't amplify what isn't there). You don't always want those really low frequencies that get cut to be amplified to the point of distortion - it will just reduce the amount of note separation and add noise under gain, which can sound like ass really fast, especially when you add in bass and drums to the mix.
That, and they're great for leads because they shift the tonal center more towards the upper midrange while taking away other frequencies (lows and extreme highs) that would be competing with other instruments in the mix, and that little bit of compression that brings out harmonic overtones will make leads scream. This all helps the guitar become more discernible in the mix.
Now, you don't want to boost too much, or you'll really run out of headroom fast (things would just become a thin distorted mess with more noise), and all of the transients of notes will get squashed which will really neuter the guitar sound/take away all of its punch. Boosts?EQs/etc all have their trade-offs, and there's plenty of times where they can be used tastefully, and other times where they'll just ruin the sound.
The modeling amps are completely different and not really comparable. Those amps are designed to "model" iconic amp sounds after they've been recorded. These sounds are already boosted/EQ'd/processed to hell and back, and extra processing isn't going to help anything. Also, modeling amps try to amplify a sound (whether it's clean, crunch, high gain, etc) with as little clipping as possible. This clipping is not the same type of overdrive/distortion we like. It's harsh and unpleasant. Less headroom before you even get to the modeling section is just going to add more unpleasant clipping.
In short: Analog amps without a boost are a kind of a "barebones" sound, and OD's/boosts just color them closer to whatever processed sound we're trying to achieve. Digital modeling amps are trying to amplify an already heavily processed (boosted/EQ'd/compressed) sound. That doesn't make either design more "clever" or "better" than the other - they have different approaches for different purposes.
Also, I think this thread probably belongs more in either the Amp room or Pedal room. Not a big deal. A mod will probably move it eventually.