Online community guidelines detail the types of behaviors that are and are not appropriate on your community. They include things that some might consider obvious, but they also include items that are somewhat unique to you and your community. I liken an online community to a country. Each country has culture, laws and social norms that make it different from every other country.
I believe user guidelines are pretty important. Let's discuss why.
They Level the Playing Field and Mitigate Uncertainty
The absence of guidelines leads to people making up their own or thinking that anything goes. With no official set of guidelines that is applied consistently, you instead get everyone else's interpretation of what they believe "should" be alright. In some cases, this may also lead to them defending their interpretation against the interpretation of another member. Certainly, that can get messy. But the uncertainty of it all is a big turn off because most people aren't going to be the ones bold enough to try to set their own guidelines. They are more likely to join a timid group that is unsure of what is acceptable and, as such, just tries to play it as safe as possible. For some, this will just mean that they decide to go elsewhere, to another community that has a more established structure that matches with what they are looking for in a community.
Good guidelines, like good rules and laws, tend to help us to all participate on a level playing field.Once we know the ground rules, we are free to express ourselves in a manner that respects them. Some may view guidelines as restrictive but, just as much, they free people up because they no longer need to worry about what might or might not be OK.
(Some) People Look for Them
Now, I know what you are thinking. "No one reads them." You've probably heard someone say it before. However, the truth is that some people do read them. I'm not saying it is a lot of people, but some members do seek them out, especially new members, but also veterans in search for a refresher. Those people who do look for them are trying to do the right thing. They want to make sure that something is OK. This is why it is not only important to have guidelines, but to link them in visible areas, such as your header, footer, near areas where contributions are made (like reply boxes), in staff member signatures, etc. If everyone knows where they are housed, it maximizes the usefulness of your guidelines.
They Serve as a Vision Statement (of Sorts)
Community guidelines are a living document and, more than a set of rules, they speak to the type of community you are and the audience that will most appreciate what you have to offer. No community is for everyone. Even a community for everyone isn't because not everyone wants that. Your guidelines help to demonstrate this and help people to come to that understanding sooner. Vision statements tell people where an organization wants to go in the future. Your guidelines should speak to that. And if a part of your guidelines ever stops speaking to that, you should change it.
They Give You Something to Refer To
This may be the biggest one. If you try to apply any sort of standards to your community and you don't have any sort of public guidelines, it feels unfair and arbitrary. How can people know that something violates the guidelines if there aren't any guidelines? If you remove content without guidelines to refer members to, then it looks like you are removing content based on imaginary rules that only exist in your head. It doesn't inspire confidence. When you remove content, you notify the person who posted it and tell them why. When you tell them why, you include a link to your guidelines, so they can see the publicly posted standards that all members must adhere to.
To sum it up, having guidelines isn't about making sure that everyone reads them. Instead, they exist to serve as a point of reference, so that everyone knows what standards exist and what is expected. There is no guess work, there is no mystery. This helps to create an environment of honesty and fairness.