Do what works is meant to create a firm but flexible foundation for Arbital policy. The aims include making the system self-correcting by causing people to flag bad policy, causing everything to tie back to actually valuable goals, and avoiding pitfalls of other community structures. It takes inspiration from Wikipedia's Ignore all rules and Eliezer's Nameless Virtue.
Don't we need hard rules to stop bad actors?
The kind of user who badly violates good practices of any kind tends not to be the kind of user who reads policy pages. It is pointless to attempt to guard against them with strongly worded policy %%note: And harmful/discouraging to another class of user, those careful enough to check the rules. And staff checking for what tone they should use towards users.%%.
Instead, it's important to give the people who are part of systems are in place to prevent harmful activity freedom and affordance to act quickly, enough training and selectivity to reliably tell obvious from non-obvious cases, a playbook of well-thought-out responses to different classes of situation, and a culture of openly discussing how best to respond with others who understand and can change the guidelines which are used by the system with the real goals always in mind.
Content
Attempting to codify exactly what content should conform to seems like a fool's errand, especially since we don't know. Explicitly encouraging people to do what works (aka. create the version of the page which is good for readers) rather than pointing them at dozens of pages of guidelines they won't read should cause them to optimize for something closer to the right thing. We should, of course, offer guidelines and advice for those who genuinely want it, and link to it heavily. Just, not elevate it to required reading.
Why not a more legal structure?
Many sites create an elaborate web of rules outlawing specific harmful patterns of behavior, and task people in positions of power (often the most productive early contributors) with the work of implementing the rules in a consistent and fair way.
However, no humanly readable set of rules can adequately capture and regulate the complexities of human interaction, and there are always awkward edge cases.
So there are people who are disruptive, and those with authority/responsibility to deal with it have a strict and hard to change rulebook, and complex, slow, tiring processes. This often goes badly, setting up all sorts of tensions, tying the hands of staff, causing people to be acted against too strongly and too weakly in different cases, both of which harm the community. There is a place for legalistic, careful, deliberative, systems, but it should not be the system called upon for most issues which come up.
By tying the community rules explicitly to "do the thing which makes more healthy community" there's significantly more freedom on the part of staff to act, and significantly more keeping them pointed at what matters (so long as the group self-polices well, which needs to be designed for socially/structurally).
Isn't this obvious? Why would we need a policy to say "do the thing that works"?
Yes, it should be obvious that policy should serve a purpose linked to a real good. However, saying obvious things is fine, and if you don't have a clear foundation people tend to take an assortment of important general guidelines there are and turn them into foundations, which are then harder to dispute even when applied in counterproductive ways, or adjust when they turn out to cause problems.