I dont really see what the bike being a hybrid has to do with it. Some hybrids are so similar to road bikes that it surely can't be much of a stretch to figure out why somebody might care about speed.
You mentioned fitness reasons, which is why I track my speeds, so I wont go into that. But even if somebody is simply riding for the joy of it, it can still be neat to watch your average speeds increase over the months. Data feedback can also lead people to hop on their bike and strive for a new best, just for fun!
Anyway, I suspect this thread was originally started so people can get an idea of what is common on this bike type. Going from my wifes 7.1 FX to my 7.3FX for example got me 1.5mph. Good info to know.
To each his own I guess. For me, if speed were the important thing I would just ride my road bike. They are much faster period. I have no idea what is "common" with this type of bike. There are too many variables involved. I have four hybrids and they all roll differently, although I do know which one is the fastest for me. Some of these average speed reports on a hybrid are simply amazing. For instance, 18mph is somewhere around 30kph. On an upright bike not designed for out and out speed that is simply amazing.