Andrea, I agree. But depending on where you are, "finding a good dev" can take months, more if you're looking for a partner. And it's easier to attract talent with a working prototype than with ideas around a beer...
If you want to be an awesome business guy, learn how to code so you can talk with your devs more effectively. But for the sake of your project don't try to replace them :-)
The other thing is that in tech startups, being the "crunch the number and shit slides" guy can be quite frustrating. Knowing how to put up a prototype yourself is pretty satisfying!