Often, orphan pages will not appear in users’ search engine results pages (SERPs). This is because the search engine crawlers that are responsible for finding and indexing pages (and serving them up to users in the SERPs) navigate websites in large part through the internal linking structures that are in place on your site. (Note: search engines also use the information provided in your sitemap to discover new pages).
In short, a page without any internal links pointing to it is less likely to be crawled — and therefore, less likely to be indexed and found by potential visitors.