The UI came out a long time before computers existed in any form. Many machines were made before computers, and the study of user interfaces came with them. You wouldn't want a steam turbine with a bad user interface.

UI is almost never the most important aspect of any application, although it is very important. The only time it is the most important thing is where it is all there is. Normally the function of a device is the most important aspect. My washing machine has a bit of a rubbish UI, but it is still a very good machine.

To call something eye candy indicates it has no purpose (unless it is used as a derogatory term about a piece of user interface you don't like). If what you call eye candy helps non-technical people feel a connection with the device, it has a valid UI purpose so it cannot be described as eye candy.