Instead of having to pay $600 an replace your entire phone every 6 months to keep up with the latest and greatest, you will be able to just swap out a module (CPU or screen or something else) and update your existing phone. Project Ara isn't a hack either. They're thinking through all the elements quite carefully including the ability to hot swap modules and using magnets to hold everything in place tightly.
It's bold and ambitious and runs directly contrary to the move throughout the IT industry (phones and computers) towards tighter and tighter integration with less replaceable components. That the geeks and hackers will want it is unquestionable, what's really unknown is if anyone else will care.
Because if you look at the mass market, there are very few products that fit this mold at all. Generally people buy a phone, keep it for 2+ years and then buy a new one. They don't care if it's not the latest and greatest. The same is true of computers, and tvs, and DVD players.
There are significant potential costs to a modular design (both in monetary and design terms) too. There's a real risk that a modular phone will end up being more expensive and less capable in terms of raw power than an integrated equivalent.
I kind of want this to work, just because. But in truth, I'm not sure I'd buy one.
Source: The Verge