I leave my kindle at home most of the time. Sometimes, while at work, I wish I could take a look at a particular passage of a book. The iphone app is OK for waiting rooms, but is definitely not ideal for more than casual reading. Having a mac version would be very helpful for me.

No, the point is to make the Kindle hardware a good-enough substitute for a book that they can use it as a loss leader to grow their digital book business.

Or, the iPhone/PC/etc versions of the Kindle reader make every Kindle sold even better, giving you more reason to buy a Kindle and Kindle books. I.e. even if your kindle breaks you can still read the books you bought.