In the case of the iPad, the thinness (and corresponding weight reduction) makes it more usable as a tablet, especially with the 13" model. It also fixed - or at least, mitigated - a few problems with the keyboard attachment.
Granted, that attachment shouldn't exist, but that's a different problem whose root cause is "Apple expected the iPad to replace the Mac like the Mac replaced the Apple ][".
Granted, that attachment shouldn't exist, but that's a different problem whose root cause is "Apple expected the iPad to replace the Mac like the Mac replaced the Apple ][".