SEO is a technique which can improve your site's traffic. It helps Google easily understand your great content. It can help your content to rank better on Google. Also on other search engines. That may bring some or a huge amount of traffic. Traffic is what we all want. Some SEO tactics are SCHEMA, Proper Headings, Table of Contents, Formatting etc.
Web cache is really useful to speed up your site. It helps users to get through content quickly. It also reduces server load. Server does not have to look for the database each time. Hence faster speed and less TTFB. Which brings more traffic and revenue.
Robots.TXT file is located in your site's root. It helps bots like Google to find your great content. And also crawl it. It guides bots What to index and What not to. You can use it to disallow bots to crawl secured, private or any file. Almost all bots read robots.txt before crawling a site.