twistr25 pretty much summed it up. The "commercial" kegs have a Sankey connector on top. So, there is the need for a Sankey coupler for your keezer or kegerator. The biggest reason that homebrewers use the Corny kegs are their ease of use like twistr said. They have a much larger opening on the top of the keg that is a lot friendlier in terms of cleaning. Those tops are also a lot easier to open and close. Sankey kegs have a MUCH smaller opening (on the order of about 1.5" or so) which really hampers your ability to clean the keg easily. Also, and this may be the deciding factor for some, the keg spear on sankeys are held in by a snap ring that can be extremely difficult to remove. Once that's removed, the spear needs to turn a whole quarter turn (on most kegs) which is difficult to do with out the proper tools. So, that's a little more in depth explanation to what twistr said. You certainly can use Sankey kegs, but most don't thing they're worth the effort.