The purpose of the conical is that you can dump the yeast from the bottom making secondary unnecessary
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Exactly. This is the beauty of the 3-in-1. Now that conicals have been around for a while, home brewers have been enjoying the luxury of dumping yeast / trub and using the conical as a secondary. That saved a big step (all the sanitation and risk of oxidation, time commitment etc.) incurred by that transfer step.
With the 3-in-1, as you know, we can now BOIL and CHILL in the conical. Now we've gotten rid of that transfer step (boil kettle to primary).
The BIAC takes it all the way. Mash, boil, chill, primary, secondary all in a single vessel. All the sanitation steps, brewhouse efficiency detractors, time commitment that went with those steps is also gone.
I found, after getting fairly serious about home brewing, this streamlining of the process without compromising other elements has really made things more enjoyable all around.
Of course, you have the freedom to transfer at any time just like any other vessel. I have considered this, say for a sour or something I want to let age in secondary / tertiary for some time. Though I haven't done it, if I were to, I would personally transfer with CO2 to whatever the next vessel I want. For that matter, you can secondary in a keg!
We are only limited by our imaginations!
But the BIAC is pretty heavy, so gone are the days of setting the secondary carboy up on the countertop and siphoning into the keg. While you'll be able to siphon, since it is relatively low to the ground, for a 5 gal keg, you will need to be able to push the beer out to fill the keg. A 3 gal keg is no problem...or for that matter, a 5 gal keg when dealing with a 10 gal batch...
Just some general thoughts which may or may not directly answer you question...