Spirit of the Law

Think about it this way. Language is conveyor of information, but it is, of course, imperfect. You have ambiguities, misinterpretation, definition issues, and so on. For this reason, rules are also imperfectly conveyed. You can try writing down every single restriction and define every word, but this is impractical. The consequence of this imperfection is that people can always find loopholes in the written law, questioning proper definitions of ambiguous words and make use of them to get around the law.
For this reason we differentiate between the letter and the spirit of the law. To follow the law "to the letter" means obeying it according to the literal interpretation; while following the spirit of the law is obeying it according to its intent, which may not be identical with the literal interpretation.
Example? Now that's difficult because formal laws are worded as such that loopholes are very difficult to find. Let's talk about grammars instead.
"You can't begin your sentence with a conjunction."
"But, I don't understand, Sir."
"Mr. Finn, you just begin your sentence with a conjunction. Stand outside."
I remember red markings on my English composition papers highlighting the offending conjunction-initiated sentences. But, of course you know that this rule is broken all the time in all kinds of writing. Why so? This is because conjunctions are supposed to conjunct clauses in a sentence. And then, why is it alright to break this rule? Remember that we are concerned with the spirit of the law. The purpose of grammars is ultimately clarity.
Compare

I remember red markings on my English composition papers highlighting the offending conjunction-initiated sentences but of course you know that this rule is broken all the time in all kinds of writing.

and

I remember red markings on my English composition papers highlighting the offending conjunction-initiated sentences. But, of course you know that this rule is broken all the time in all kinds of writing.

The first sentence is too long and this obfuscates the meaning. It is better to put a period to give the reader a break. The use of conjunction to precede the sentence indicates that the sentence that follows it still continues the idea from the sentence before. A good writer does this: giving the reader bite-sized information and not confusing him/her with long-winded sentences; he prioritises brevity and clarity.
You can see how are letter and spirit of the law different. Otherwise you can see it as the rule of clarity overrides this rule.
If you know a little bit of jazz, you may know that jazz is a genre that doesn't obey the rules. Before jazz came about, there are certain ways melodies sequence and group together. In jazz, however, improvisation is imperative. This means there is no one fixed way to play a piece: a performer is free to interpret and tweak melodies, harmonies and time signatures. The spirit of the law? To produce nice sound; it's that simple.
In forensics, there is a principle called corpus delicti, which is translated as 'body of crime'. A British serial killer, 'Acid Bath' Haigh, infamously mistook this principle. He thought that he could not be convicted with murder without the bodies of the victims, so he dissolved their corpses in acid bath. But of course corpus delicti is not to be taken literally. It refers to evidence that the crime has taken place. Corpus delicti is not even necessarily tangible. Circumstantial evidence is often enough to convict. This is not really about the spirit of the law, but it illustrates how laws should be understood clearly.
Lastly, there is a reason to break rules. In the grammar example, you have to be clear about the big picture. That grammars exist as a guide, and occasionally breaking them for the sake of clarity is alright. In the same way, if you encounter rules that don't make sense, stop and think about what the spirit is; see the big picture.
I recommend reading How to Break All the Rules by Dustin Wax down at Stepcase Lifehack:

There's a scene in Kurt Vonnegut's Bluebeard that sums up perfectly this approach to the rules. Rabo Karabekian, an artist reknowned for his giant canvases covered with single colors of household latex paint applied with a roller, is talking with his friend Slazinger in his studio:

"Tell me, Rabo," said Slazinger, "if I put on that same paint with the same roller, would the picture still be a Karabekian?"
"Absolutely," I said, "provided you have in reserve what Karabekian has in reserve."
"Like what?" he said.
"Like this," I said. There was dust in a pothole in the floor, and I picked up some of it on the balls of both my thumbs. Working both thumbs simultaneously, I sketched a caricature of Slazinger's face on the canvas in thirty seconds.
"Jesus!" he said. "I had no idea you could draw like that!"
"You're looking at a man who has options," I said.

For the "wild child" who just can't be bothered to learn the rules, because they were meant to be broken anyway and because his or her creative spirit is too strong to be held down by rules, man, there are no options. There is only a string of broken rules and all the misunderstanding, chaos, and incoherence that goes along with them. The master, though, knows that the rules are not only options, but usually the best options. And when they aren't, s/he knows. S/he has in reserve what Karabekian has in reserve: true mastery.