A hack is a domain like Awso.me, Blo.gs, or Gam.es. The keyword on it's own makes no sense, add the extension and it spells out a word or phrase. A domain like Office.pro, which I hold, isn't a hack, professionals work in an office, the keyword Office fits the extension .pro. There are two types of fit; association and semantic. For example, if it was possible to have the domain bread.butter, that would have association fit whereas garlic.butter would have semantic fit.
I hold other professional context .pros like Studio.pro and Salon.pro. Creative professionals like artists, software developers, and film directors work in a studio; beauty therapists and hairdressers work in a salon. I also hold Tool.pro, professionals use tools of various kinds so it makes sense.
Fits adds desirability and brandability to an alternative extension. For example, in .info, I hold information concept keywords like Rates.info, Prices.info, Coupons.info and Savings.info. These aren't hacks, people want these domains because of the fit, look at how many bids they have on Sedo between them. Prices.info has more bids than Prices.net because Prices fits .info better than it fits .net.
Fit is critical for alternative extension value, the highest selling domains across alternative extensions illustrates the point; Travel.info sold for $116,000 in July 2007, RingTones.mobi sold for $145,000 in October 2007, Fishing.net sold for $52,500 in January 2008, and University.org sold for $100,000 in October 2006.
You could have ProGame.com or Toolpro.com or you could opt for Game.pro or Tool.pro, I prefer the latter, they are shorter, quicker to type, and more striking. You asked why people register "the extension" .pro, then added .pro is a branding suffix, not an extension. It's actually both, that's why some people like it. It's not everybody's cup of tea, the vast majority of people like the security, conformity, and mindset saturation of .com. However, I prefer the impact, brandability, image, and originality of .pro.
From what I knew .pro is available exclusively for: Medical doctors (.med.pro), Attorneys (.law.pro), Certified public accountants (.cpa.pro)
In September 2008, .pro restrictions were changed so that all professionals in all countries could register at the second level, ie keyword.pro. The biggest registrars are Encirca, Network Solutions, Domain People, OVH, and Gandi. It costs about $20 to register, you self-certify your professional credentials when you register, then you have a certain number of days to submit your professional license number, regulating body and other details. Professionals can register any keyword, they are not tied to keywords relating to their professions.