Beta testing simply refers to the process of having some typical users test the software to try to find errors before it is released to a larger audience of users. Similarly, beta reading of a manuscript is a way to have typical readers evaluate the quality of the writing before it is published for the whole world to see. It is cheaper and less unpleasant to correct errors and improve the product before it is released.

The earlier you find and correct a problem, the less it costs and the less effort it takes. When publishing an article or a book, the manuscript becomes more and more difficult to change as it goes through the publication process. For instance. once the pages are typeset, you must avoid changes that would affect where page breaks occur. And after it is published, of course, it is what it is, until you reprint it (which you might never do). So use beta testing (software) or beta reading (written works) to make the product as good as you can before releasing or publishing (or even posting) it.