__________________ Windows 10x64 | AMD Ryzen 3700X | ATI FirePro 2100 | Marian Seraph AD2, 4.3.8 | Yamaha Steinberg MR816x "If I can hear well, then everything I do is right" (Allen Sides)
And isolation. Just because might as well keep the room sound separate. Some people of course mic further back precisely to do the polar opposite and get some room sound into the track.
Simply because it usually sounds better with more direct sound and less reflected room sound.
And if the whole band is in the same room yes, to maximize isolation so you can control the mix.
Acoustic instruments are often recorded in a "nice sounding" room with the mic positioned for more reverb/room sound or multiple mics may be used to pick up the direct sound and room sound.
If you've ever recorded a live band from your audience position you've probably noticed that there is too much reverb and it doesn't sound very good. The amount of reverb that sounds great coming from all directions live in a music hall usually sounds unnatural coming from a pair of speakers in your living room. (In a live situation, the crowd noise also "sounds worse" then heard played-back that it sounded live, so again separate mics are often used to separately record/control the applause/audience noise in the mix.)
But put your speakers in a music hall and play a recording with very little reverb and that recording might sound great with the natural reverb in the larger space.