If an online magazine or publication wants to have any sense of consistency or standard in its content, then a style guide is needed in order to achieve that. A style guide, similar to that of the AP Style Guide, is a set of guidelines your publication adheres to when it comes to industry terms, company names, and other words relevant to your niche.
For example, is it "ebook," "e-book", or "eBook"? An online magazine without a style guide may have all three littered throughout its articles, and that just looks sloppy to readers. However, a style guide would determine which one is correct for YOUR magazine, and would ensure that "ebook" is the one consistently used throughout the publication.
Creating your own style guide is incredibly crucial for niche online magazines since the AP Style Guide doesn't get into those niche categories. Most online magazines have to deal with city names, for example bu tit's unlikely AP has any ruling on whether it's "shipbroker" or "ship broker", or "coworking" versus "co-working" versus "CoWorking". Keeping that niche terminology consistent will help your magazine be the go-to source for your community as well as maintaining sense of professionalism in its content.
The nice thing about style guides is that they aren't set in stone. Entries can be added, removed, and changed. The purpose of it is to keep your editors and writers following the same rules. You wouldn't want to waste time going back and forth on a few edits simply because one person things it's "cyber crime" and the other thinks it's "cybercrime". A style guide would prevent the needless editing and confusion.
Style guides don't just have spelling and grammar rules, but factual rules as well. Do you know what NASA stands for? How about UAW? The AP Style Guide will tell you, but having your own can also define what SaaS, VPN, and IP mean. If your readers know, and you get it wrong because you didn't have a style guide to verify the information, then prepare for your readers to call you out on it.
Online publications ought to develop the style guide before publishing their first post or issue, but if that's already happened, then better late then never. A style guide will add consistency and standard to what your publish from here on out. All legitimate publications have one, and it's not necessarily something that needs to be shared with readers. It's to make the publications better at what it does.