The
module builder customizes the operational aspects of a module by adding, removing or reorganizing fields, sections, sub forms, links and buttons.
On the other hand, the Canvas builder customizes the visual aspects of a detail page. Using it, you can reorganize fields and sections, include only the required action buttons and make the page look attractive and artistic. This way, the record detail page is more organized and interesting. Also note that, just because a field in the Canvas builder hasn't been used in the design, it does not mean the field has been deleted from the module. It is still available inside module, and not displayed in the Canvas view.
Similarly, if you want to further add new fields into your Canvas design, the fields first needs needs to be added to module builder. Once added, you can go back to the Canvas builder to include it on the page.