When you make a regular post on Instagram you are constrained by a certain number of slides and a certain number of characters in the caption. If you try to add a lot of words to your slides, there is a chance they could end up small and unreadable. If you try to say too much in the caption you will have to break and reply in the comments, which is also not ideal.
Your feed posts will inform your Guides, and vice versa.
Stories are limited by only being available on the feed for 24 hours (although you can highlight them on your profile afterward). It is not an ideal way to convey large amounts of information. For large amounts of information, a Guide is better.
Guides are effectively a kind of blog post, but one that is within the Instagram platform. This makes it the perfect way to expand on the information that you're providing in a post, in a clear and informative way for your audience to enjoy.
You can collate all your posts around a certain topic or theme in one Guide making it easy for your audience to find the information that they need.
Some posts do not need to be backed up, standalone posts still find any do not need to include every single post within a Guide. However, they can be very useful, especially if you are supplementing your posts with those of other users that verify and add to the value of your posts.
The key to Guides, as it is with all forms of media, is finding the sweet spot. It may take some experimentation, but soon you will learn which kind of posts require Guides that will best work for your audience; there is not one unique solution to this.
Another method is analyzing what engagement is like and finding the responses that your audience has to your posts and stories is the best way to know which kind of posts require supplementary information.
For example, if you are getting a lot of questions on a certain thing perhaps creating an FAQ Guide would be perfect for you or just the response to one post.
As for all social media, the beauty of Guides is that they can be customized to you and your audience, based on your needs.