Your SEO keywords are the keywords and phrases in your web content that make it possible for people to find your site via search engines. A website that is well optimized for search engines "speaks the same language" as its potential visitor base with keywords for SEO that help connect searchers to your site. Keywords are one of the main elements of SEO.
SEM results show to a select target audience. SEO results do not. While successful SEO and SEM strategies are driven by a plan to connect with a select audience, you can only specify that target audience through SEM. Through SEM, you can (depending on the publisher) select what audiences you want to see the search results by assigning filters based on age, location, income, habits, and more. Through SEO, you cannot specifically choose who will see your search results.
Most people search on mobile devices – You don't need statistics to show you that in the past few years the online mobile market has exploded, overtaking desktops years ago. Optimizing websites for mobile browsers is critical if you want to rank well in search engine results pages. If you’re unsure how your website measures up, enter your site’s URL in Google's Mobile-Friendly Test.
Additionally, there are many situations where PPC (a component of SEM) makes more sense than SEO. For example, if you are first launching a site and you want immediate visibility, it is a good idea to create a PPC campaign because it takes less time than SEO, but it would be unwise to strictly work with PPC and not even touch search engine optimization.