Lets consider something like the right to take sick leave. There is no strict definition of what "sickness" is. It is something that YOU feel impacts your ability to work. Maybe a bad nights sleep, sore back, headache, runny nose, bad tummy, etc. There's no need to provide any amount of proof to an employer when taking a single day off sick, and they need to pay at least the statutory sick pay, many companies pay full.

Most people agree that this rule works well in practice. Most people don't take a sick day because they don't feel 100%, even when its within their rights. If everyone decided to take Monday off because they're sick, that would be possible within the law, there's no need to provide proof you're sick. Yet, if that happened, the country would grind to a halt.

Now ask yourself, how could you implement that without eroding someone's rights and without introducing more friction?

Sometimes, you just have to assume that people are honest and won't take the piss out of the system. Or you have to introduce friction to discourage the exercise of their rights. The friction for sick leave is that you usually have to notify your employer ASAP and that you may have to fill out a return to work form.