I think the intention is to measure the adapter itself independent from the CPU/overall system.

Besides, I can’t think of a typical single threaded application that would use those data rates, can you?

File transfer and storage (Dropbox, browser download, rsync, scp, NFS/SAN etc) is a classic use case that can utilize all the bandwidth you have and typically uses single streams between client and server.

Steam downloads

Steam download rates are throttled based on how fast it can actually install the game so it’s a bit of an outlier