Mozilla decided to create B2G for several reasons, but one of them is because most of the world's population in the near future will be accessing the internet from a phone or _maybe_ a tablet, not a full-on laptop or desktop. And people using phones or tablets don't install non-default web browsers, statistically speaking, not least because storage is pretty limited on phones, so if Mozilla wanted to be in the market at all it needed to be shipping the default browser on a phone people would use.
There was also the reason of wanting a phone/tablet marketplace without vendor lock-in, which requires apps to be portable between phones from different vendors. That's where web apps come in. And yes, apps that you can move to your new phone even if you get it from a different manufacturer are intrinsically more valuable than apps that you lose if you move from iOS to Android or vice versa.
As for why you'd go for Firefox OS over Android, one answer is it performs better on limited hardware (think a phone with 256 megs ram, and yes, it's pretty hilarious what counts as "limited hardware" nowadays). If you say you're not likely to be buying a phone with those sorts of hardware specs, then you're not the target market. Remember what I said about "most of the worlds population" above? Well, the total population of Europe and North America is about 25% of the population of the world. The other 75% is not out to buy $600 phones. Neither are parts of Europe and North America, of course...