Pages are used to organize and present information and media on the primary and subpages of your site. Page titles, settings, attributes, attached template and place in the page hierarchy (parent/child) all play important roles in creating the navigation and flow of users through your website.
Posts, on the other hand can be included in various (often dynamically presented) features throughout WordPress such as in blog grids, news listings, or press releases. Tagging and categorizing posts can enable targeting specific posts to specific features or presentations. Embed posts into a page with widgets or blocks like Recent Posts, Post Grid, Post Carousel / Slideshow, etc.