Before July 1996, learner drivers only had to answer a few questions on the Highway Code at the end of their practical driving test. Now, there is a computerised touch-screen multiple choice test and a hazard perception test which you must pass before you can even apply for your practical test.
If you want to be a safe driver, you need to know what that involves. And because the consequences of bad driving can be lethal, there are many laws and rules of driving that you should understand. All drivers, indeed all road-users, should apply the same rules, otherwise there would be total chaos out there. You need to make sure you know what you're doing, and what you can at least hope for from other road users.
A great deal of what we call driving theory is common sense which you may know instinctively. However, before you can drive a car safely on the road, you MUST know and understand many other things like Safety Margins, Hazard Awareness, Vehicle Handling and the Rules of the Road.
Amongst the reasons for so many crashes and deaths on our roads are drivers ignoring the rules or failing to anticipate hazards. A sound knowledge of driving theory will help you and others survive to enjoy a safe and enjoyable driving career.