The concept is to knit 3 layers (flat) at one time, creating a bag with pocket all at once (with one strand, one color of yarn). No in-the-round knitting involved. Once the pocket is as tall as desired, it’s bound off, and the rest of the bag is traditionally double knit - still one strand, one color.

Note: this whole thing would go faster if two strands (three???? - don’t think so) were used. I haven’t tried it to see if it’s a straightforward technique.

I imagine this has been done before, but I’ve “unvented” it while noodling around with another double knit conundrum I had. At any rate, this is only swatch-sized, with barely room for my battery backup & headphones. Eventually, I hope to make a practically sized version and write up a proper pattern.

Tubular caston. Should have used smaller needle. Instead, I used my weaving-in thread to tighten it up a bit at the bottom.
One round double knit.
Increases for pocket.
4-pass method for triple-knitting (3 layers).
Bindoff pocket purlwise (catch edge stitch of back at same time). “Center, center, end, end.”
Double knit remainder of bag in 2x2 knit (next time, have mult. of 4 +2 each side so it starts & ends with k2).
Bindoff in pattern.

The main reasons I can think of to do this instead of in-the-round:
Just cuz it’s a challenge
If one prefers flat knitting
It makes a nice seamless pocket (though that can be done by double knitting a pocket onto a regular flat or in-the-round piece as well)

Another application I can think of is a small portion of a double knit blanket - adding a pocket into which the blanket can be folded. Again, if you do a single-knit blanket, you can just double-knit the pocket on

Double knit pockets require a technique of their own. It’s explained well by June Hemmons Hiatt in “Principles of Knitting.” Lucy Neatby has addressed it as well.

Photos
01 completed
02-04 instructions only I can interpret
05 three layers on one needle, knit flat (one selvedge at each edge - could omit them)
06 separated layers to show the interior
07 interior with items inside
08 bound off pocket
09 working on 2x2 double knit portion
10 separated two layers
11 bound off in pattern
12 the back
13 the front from inside (& shows interior of pocket - reverse stockinette)
14-15 the back from inside