Just like cars, computers are a means to an end. However, cars are an established technology, and as a society, we've decided that there are some things that must be expected of a driver - the ability to operate the car safely, and understand what is likely to damage it (Putting it into reverse gear while doing 70 on a motorway, for example is as bad for a car as switching it off in the middle of a defrag is for a computer). In most countries these things must be understood before someone is allowed to drive a car unsupervised. A computer, however, can't kill someone, so the need is much less urgent.1
However, how many drivers can you think of that can't refuel their own car? And what would happen to you if you couldn't change a tire (even after consulting the manual) in the middle of a desert? Computers, on the other hand, are still such an immature technology that it's perfectly acceptable to not understand the concept of a hierarchical file system, or that different drives/mount points correspond to different devices, some of which may not be physically 'in' the machine, and yet still operate one in the course of your work. Society is much more forgiving of someone that can't comprehend that more than one program can run at once, or that doesn't know the difference between memory and backing store, than someone who changes lane constantly, or who can't parallel park in less than 27 actions.
At some point, society as a whole (as opposed to just disgruntled techies) will expect some basic level of competence with computers, in the same way as they do with cars. (Indeed, there is a scheme being piloted by the EU called the ECDL - European Computer Driving Licence). We can only hope, huh?
Some of these 'car tasks' are above and beyond for the mere mortal, but others are expected, to the extent that they are necessary to get a drivers license:
Turn the car on vs Turn the Computer on ; Go for a drive vs Operate software
Obvious, simple. Anyone can do this, to some degree of success. I managed to turn the car on when I was three, but didn't get to drive it very far: I was far more interested in the cigarette lighter. If someone professes to be a 'driver', but can't do these things perfectly, they'd be laughed at.
Put gas in the car vs Manipulate files
If you can't put fuel in your car, eventually it will stop working. If you can't work with files, eventually the drive will fill up with dross, and the computer will stop working. With a poor understanding of how to work with files, it becomes more likely that important stuff will become 'lost'. Knowing to/how to backup files is vitally important in even the most mundane uses of a computer, just as filling up a car is vital to its continued operation
Check the oil vs Run ScanDisk/Norton Utilities
If you can't/don't check the oil, you run the risk of the engine seizing. If you can't/won't run some kind of 'checkup' program, and act sensibly on the results, you could lose valuable data. Unlike the earlier tasks, however, it's borderline acceptable (If you have the money) to get someone else to do it. The techies in PC world will laugh at you behind your back, just as sure as any mechanic would laugh at someone who can't check their oil
Change a tire vs Install Software ; Change the oil vs Install a new operating system, install a new card
These tasks are the first ones that actually need some skill or specialised tools to perform - With careful manual reading, and household tools, it's perfectly possible to change a tyre (All the necessary tools are supplied with the spare tyre), or even the oil. Likewise, installing new hardware or software is (usually) a case of reading the manual and following the prompts. Easy. Ish.
Tune the engine vs Diagnose hardware faults ; Drop the engine out and rebuild it vs Build new hardware
No-one that uses a car is expected to be able to build an engine. If your car goes wrong, the accepted behaviour is to take the car to someone who can fix it, and pay them. It's possible to save money by doing work on your car yourself, but it requires skills uncommon in those who don't fix cars for a living. Same (at the moment) with hardware issues. I have a degree in software engineering, and it's very rare for me to build even simple hardware from components. I've never rebuilt an engine, and I'll wager that Damon Hill hasn't either. There are some things you leave to the experts.
But when you're leaving everything to the experts, there's something wrong.
1 - While a computer can't kill someone, incompetently-maintained computers connected to the internet quickly get r00ted, and inconvenience everyone else by acting as a platform for DDoS attacks, the sending of spam, etc.