Well, they're all about pacing.
I've aimed to write fast-paced books, each one with its own standalone plot that is resolved by the end of the book. At the same time, each book is working together with all the other books to progress the overall story of the series. The biggest challenge has been in balancing those two elements.
The overall story builds on events of the past, and so earlier drafts of the novels included flashbacks. The problem was these flashbacks were weighing down the story, screeching this smooth forward momentum to a halt. When I realized this, I considered ditching the flashbacks, but cutting them out entirely would have flattened the layers of the plot I was trying to unfold one by one.
After a lot of thinking and pacing across my living room—there may also have been some Swiss chocolate involved—I came up with the idea of expanding these flashbacks into full stories and putting them into companion books between the main books. This also allowed me to dive deeper into certain characters, exploring the experiences that shaped and motivated them. For instance, I could tell the stories of why Everett distrusts magic and of how Ariella came to fear magic (two of the tales in Menace: Book 2.5, the cover shown above). And I wasn't limited to only events in the past. Some of the stories play out in parallel to the main books.
Not only did moving these stories from the main books to the collections streamline the flow and pacing of the novels, they were also really fun to write!
So, should you read the .5 books?
I hope you do. They're meant to enhance your reading experience of the series. You won't be lost if you don't read them, but you will certainly understand the events and characters much better if you do.