Quite simply; not everyone has access to Edge or 1xRTT. GPRS is too slow for the heavier kind of work, and with WiFi popping up all over the place the potential for new services on these devices is awesome.

For example; I can make a VOIP call on my WiFi enabled PocketPC using one of many VOIP clients (like Skype or XTen). I can listen to streaming radio over WiFi when in an RCC.

WiFi offers speed that none of the cellular technilogies even get close to.

For email I use my Blackberry, as I get over 450 emails a day doing that over regular GPRS with a PDA simply wouldn't work well, unless I were to invest in a push client for the devices (like from Good Technologies). WiFi would make downloading that many emails get closer to being practical. Not to mention around the house; I use my WiFi PDA to operate my home automation equipment.