I don’t know what their specific reasons are, but looking at the meshtastic docs and comparing longfast to mediumfast, they both use the same bandwidth and amount of redundancy (for interference protection) but mediumfast transmits faster than longfast, reducing its range in exchange for faster transmissions, which also clears up the airwaves as people spend less time transmitting - and also increases the capacity of the airwaves (data rate).

One reason I can think of that you would do this is if it’s a congested area where there is no shortage of nodes to retransmit, so you might not mind shortening your range if it means less congestion and faster transmissions.

That’s just my guess though.