Is this the basic idea of "the" audio track in BIAB?
Basically, yes. The one audio track inside BB is handy to have, but if you want a lot more flexibility in your audio recordings, the included RealBand allows for multiple tracks, more comprehensive editing facilities, etc.
If so, why not just start out in Real Band since as you say it also supports the song generating features of BiaB?
Band in a Box simply has more autoaccompaniment features available. Try working up the songfile in BiaB first, tweaking everything well, and then Export via Drag and Drop and open in RealBand for recording your work along with the autoaccompaniment.
RealBand has one feature that BB does not. Since the entire song gets laid out on the timeline as tracks, rather than using the Choruses, Repeats, etc. that BB does, editing, splicing and dicing, and the ability to highlight just one area, a few bars, of a BB generated track and choose to REgenerate just that area to get something closer to what is desired is hot stuff.
Perhaps more simply: why does the record audio feature in BIAB exist?
Everybody tends to view Band in a Box from the standpoint of what they intend to do with it. If, for example, your particular idea of main usage is to make great sounding accompaniments for you playing and/or singing with the end target being a produced Recording, the myopia of the thing sets in hard. I think we all have experienced that at times. Hard to picture why someone would need some of the many features of BB unless you view that feature from the standpoint of a user who is not all that interested in making recordings, perhaps they are into the Educational or Practicing aspects of the program.
Also there are the Songwriters, who are more interested in just getting a reasonably good sounding DEMO recording of their song and feel that all the slicing and dicing available in a full-featured multitracking recording program are just too much to deal with, or perhaps get in the way of their creative workflow.
Band in a Box = Autoaccompaniment program first and foremost.
RealBand = Multitrack Recording Program first and foremost, the ability to load BB Styles, RealInstruments and such on the BB tracks provided, represents a very nice work situation and timesaver.
Consider that if using any other DAW program out there, the process of adding another RealTrack, or part of one, or getting multiple performances of a single track in order to make a Compilation track out of them would involve multiple revisits to BB, export of the track, import to the DAW program, swometimes then having to line up the newly imported track to get it in the right spot on the timeline, rinse and repeat all that as necessary. Until the advent of RealBand a few years ago, that is what we had to do. RealBand can eliminate much of that process.
Both programs are designed to work together for the person interested in producing their own home recordings.
It takes time to learn each one, time well spent.
--Mac