Writing a quality book takes a lot of time and effort and the reality for most self-published authors is that their efforts will most likely go uncompensated. Yes, there is the dream that an independently published book will take off like The Martian by Andy Weir but chances are that this will not be the case for most. In fact, most self-published authors will actually lose money after they pay for a book cover design, editing and any other fees associated with publishing. But none of this should discourage anyone from writing and self-publishing their book.
I truly believe this because people who feel compelled write and publish books have something to say. They have an idea that they want to put out there. If the idea is a good one then the motivation should be the idea and not the anticipated reward for the idea. By contrast, it is probably true that if the motivation behind writing and publishing a book is the reward then the idea is probably not that great anyway. At the very least the motivation will not be aligned with creating the best version of the idea and if this is the case then the book will most likely reflect this and not be successful as a result.
I also truly believe that the act of writing and self-publishing a book should be its own reward for any author but especially for a self-published author. Yes, there is a sense of accomplishment that comes from seeing a project through all the way to its completion. But more importantly, the process of crafting the language so as to effectively convey the idea in a compelling manner is an art form and there should be a joy in creating art.
On the other hand, art is created (at least in part) to be enjoyed by others and for this reason, there is something tragic about a true piece of art that goes unrecognized. This is one very important reason why (at least in my humble opinion) it is so important for a self-published book to be reviewed. Not only will the review provide the viewing eyes that the art form needs for its validation but the review will also provide the creator of the art his or her own validation as well. Even if the book never receives a larger readership at least the effort and the work itself has been in some small way recognized. And it is this validation that will inspire the artist to go on creating.