Let the hive mind of Engadget get that for you.
"I'm in the market for a new phone and money isn't a limitation. I'm also not partial to any particular US carrier, but here are some of the features I'd like to have: WiFi, GPS, good coverage in lots of places, push Gmail (a must!), physical keyboard (a must!), a touchscreen, decent battery life and a relatively slim body. And please, nothing that has a fruit logo on it. No offense to the fruit fans, though. Thanks!"
That space is provided to allow thumbs to fully reach and use the lowest row of keys while holding the sides of the phone. As opposed to what a Treo does, for instance, which is to place the keys within about .200" of the phone's bottom, making it challenging (at best) to move through the keyboard without restriction. The real difference is between adult men who use their QWERTY phone for serious email communications and those who primarily fire off quickie emails/text messages. Using a Treo-like QWERTY for anything more than a couple sentences is a pain which is counterproductive to business.