The number of busses a mixer offers basically decrees how many simultaneous tracks you can record in one pass before you explore the direct out feature of the mixer's individual channel strips.
Direct outs are kind of cool but do have some short comings. Number one, they bypass the mute functions on most mixers and the buss assign buttons too so if you need to mute a signal such as a noisy guitar amp or a singer who insists on talking or coughing in between their lines, a direct out will be a pain. The second short coming of a direct out is that you must manually plug a wire into a jack each time you change recording channels where-as with busses, you just press buttons!
I like using the buss system on my mixer because of the conveniences it offers like muting, group fading, group inserting, fading in or out a part without worrying about wrecking a finely set recording level on the channel fader and the additional metering that is available to a buss for more accurately setting clean, optimum levels to feed to your recorder.
I know the purists out there try to avoid busses to keep the signal path a little shorter but, in reality, buss summing amps in decent mixers add negligible amounts of noise and when we look at the biggest offenders of noise, it's usually coming from guitar stomp boxes, guitar amps, inefficient microphones and actual room noises picked up by microphones like AC units, furnaces, street noises, dishwashers and other household appliances that leak airborne and electrical pollution.
The buss is the least of your worries.
Cheers!
