Portables. Show me an ST laptop faster than 8MHz and I'll show you a PC owner who backs up his hard drive every night.
With emulation you can have a silly fast Atari portable with an 800x600 16 colour display, as much RAM as you wish and will work with your Atari Modems and printers, even hard drives (in some cases).
But, erm... I can't think of any other reason. That's why I own all the Atari machines...