Obviously it didn't pan out, but you could think of Convoy as an aggregator of small carriers. A big shipper doesn't want to deal with 1,000 small carriers but they can deal with Convoy, and Convoy deals with the small carriers.
Convoy could interface with large companies as though they had a large fleet of drivers and trucks. That "fleet" wouldn't require paying for benefits to employees, maintenance or fuel on the truck, and could scale up or down based on demand.
I would have guessed that the network effects of getting this kind of marketplace going would be the hard part. It makes me think that a big mistake in leadership was made.