Live recordings often have so much more emotion than studio tracks. The dynamic changes in volume, or the very slight rushing or dragging of the tempo are ways for the band to express the music. By playing together, they are able to match this expression to end up with a great vibe on the track. A single musician playing in a studio may be matching his or her tempo and volume to other pre-recorded tracks, eliminating much of the dynamic sound from the recording. Recognize the importance of dynamics, and evaluate your recording process to see if you’re starting your recording sessions at a handicap.
You can read the full article here: http://www.prosoundweb.com/article/in_the_studio_band_individual_dynamics_and_putting_it_together_in_the_mix/