Many of the firms that we teach courses in don't have a corporate style guide; in fact, many organizations aren't aware that they exist or even what they are. Style guides can be an essential tool in any corporate framework for various reasons: they reduce the time writers spend worrying about minutiae; ensure consistency across documents; and help standardize the organization's processes, at least in terms of writing.
A style guide is simply a manual, whether created in-house, by a consultant, or bought off-the-shelf, that indicates how documents will be written and designed. So, for instance, if everyone in your organization differs in terms of using the serial, or Oxford, comma (the comma before "and" in a list) or when to capitalize the role of someone in the organization (is it "Project Manager" or "project manager" or "Client" or "client), the style guide solves these issues. As a side issue, I'm always amazed at the amount of discussion that can arise concerning the use of the serial comma; by the way, it should be used, if only to reduce confusion.
Style guides created in-house can be especially helpful because they should, in theory, deal with exact issues that your organization and staff encounter. For instance, your organization may be unique in how it wants to represent names, dates, or even how it wishes to organize documents, all of which a style guide would address. Know, however, that writing a style guide, while seemingly simple on the surface, actually requires a great deal of time and effort to do well and effectively.
Off-the-shelf style guides are also plentiful; the issue with many of them is that they often cover so many topics that they may be overwhelming to read and use. And, if your staff isn't reading and using them, what's the point?
Finally, you can hire a consultant to write a style guide. This can be especially attractive, because a consultant should know the right questions to ask and be able to produce the style guide in less time than it would take a staffer. The consultant should be well-versed in writing such guides and understand how to survey staffers to get at the issues that the firm finds most problematic in terms of style.
Whichever route you take, ensure that your staff is adequately trained to use the guide; many companies invest in style guides, only to discover that a) staffers aren't aware that their firm has a style guide, or b) staffers don't use them because they don't know how to use them, they don't what they're to be used for, and/or they haven't been trained to look for the information of most importance to them or their firm.
We advocate style guides because, long-term, they'll save your organization time and money. Writers will spend less time worrying about and debating about the small stuff, clients will begin to see consistency in your documents and therefore have fewer questions, and editors will spend less time proofing and can spend more time working on high-level content. But again, staffers have to be aware of and trained on how to use them.
While style guides won't solve all writing issues, they can go a long way toward helping your organization streamline the writing process and spend less time sweating the small stuff.