Usually in multi-party meetings engineers from different orgs are required to have an issue discussions. Usually for multiple domains being handled in a single giant call while few attendees discuss one domain issues the others domain engineers are waiting for the latter to finish. So the meeting timings are usually long.
With multiple sub channels with the main meeting, the engineers of different orgs of similar domains can discuss their issues simultaneously and then return to the main meeting, where usually management and deadline discussions happen.
So this way technical discussions could happen in parallel and time is managed efficiently.
Note: the main meeting hosts are able to see the multiple channels/calls which are created withing that meeting and hosts of that meeting could jump into either calls to understand the progress.
Feel free to reach out, if there is lack of clarity in idea.