Can you imagine opening a store and not having a banner, signage or something telling customers what you're selling?  Of course not.  You also wouldn't create a website that didn't contain that information either.  When someone looks at your website they should be able to easily tell what it's all about.  There is a big difference between our human eyes and a bot from a search engine operator that is crawling your site in an attempt to index that site to see what it's all about.  Remember, Google and other firms want to return search results to their users based on relevance.  They want to show those conducting a search exactly what it is they are searching for.  A bot cannot see your website with human eyes so it needs to be able to tell what your website is all about by other means.  Keywords used on your site, key phrases, links and other data that are able to be show what it is you're selling, writing about, advertising, etc.  This is the goal of Search Engine Optimization.   Click this link if you have time for a quick video from Google about SEO.