I think it boils down to asking what the product is actually for, or rather, what fundamental human need does it solve? That's a rather tough answer for computers, but for cars it's easy - getting from point A to point B. Every modification to a car is in pursuit of either making the job more efficient or making the job easier/more comfortable for the customer.
I think the fact that the answer is so tough for computers is why feature creep is so common with software. When your tool can literally do anything you want it to, why say no? The constraints are actually in the mind of the customer.
This is very good point. The constraints are in the mind of the customer, but also in the UI. i.e. a panorama viewing app is not great in vertical orientation on an iPhone. There are physical as well as mental constraints in the UI of a computer.
On the topic of cars, it is true that modifications strive toward comfort, but they also strive for marketability. While a leather-wrapped dashboard may be comfortable to look at, it is primarily focused at adding something to market. This same phenomenon exhibits in software, where features are added to increase marketability, "Look! this ERP software has a social network built in!". The utility of a given feature is often perspective-based, especially when the user is not well aware of their actual needs, or the user and purchaser are not the same person.
I think the fact that the answer is so tough for computers is why feature creep is so common with software. When your tool can literally do anything you want it to, why say no? The constraints are actually in the mind of the customer.