Races never finish at the bottom of a hill though and that's a problem. I suspect I would win a lot more if they did. Right now I'm imagining a Tour de France with a Downhill Sprinters Jersey and winning it requiring inverse skills and attributes germane to the Best Climber. The Jersey itself would also be inverse. Instead of white w/ red polka dots it would be red with white dots.
That's never gonna happen though but it's not to say downhill sprints don't have big benefit, at least for me. Primarily this is safety. Bikes do different stuff at very high speed and it's way safer to be familiar and comfortable with the kinetics of what a bike does above 37+ mph. Waiting to experience this in a race is a mistake, in my view.
I'm not strong enough to reach these speeds by myself on the flats and I don't have a motorbike to sit on, so I go and find a steep hill that allows me to coast up to 30+ mph and then uncork fresh efforts from that speed. This training teaches many things.
My 13 yr old daughter Anna is a an accomplished junior alpine ski racer. Actually she's more than that. She's Champion in her own rite & a total ripper. Safety in this pure speed sport is a big concern so her training includes an annual 'Speed Camp'. Speed camp serves two purposes. It safely extends an athletes threshold for speed and teaches them the different kinetics that happen at these speeds. Basically they gain familiarity with and the skills to manage speed. This includes a respect and an awareness they can't replicate anywhere else. Anna leaves Speed Camp with confidence built on experience which makes her ski racing way better/faster and also way safer.
I think it's the same for bike racing.