On the previous page, we looked at how breaking up a compound sentence with a full stop could be used to create emphasis and voice. In this snippet, it's more to do with information flow and readability. The full stop gives the reader a chance to breathe before diving into the next complicated set of relationships.
This is quite common in non-fiction writing (like reports, essays, news articles, and you might notice that we do it quite a bit in the instructions of these lessons), because a single idea can quickly become very complex so that you need a lot of connectors to show how all the pieces fit together, but the resulting sentence becomes hard to follow because people can only hold so much information in their brains at once. So splitting a piece out can help. (See what we did there?)
Having said that, starting a sentence with a connector can be seen as bad style in some contexts, such as formal academic writing. Sometimes you just have to forego a connector in the name of style and readability.