SEO or search engine optimisation is a technique that allow you to rank good on web search result.
What is the need for SEO for your website?
See when you do any google search for your query then you can easily see that google produces thousands of results based on your search, some of them is relevant to you while others are not.
Generally it is seen that any user just go to maximum 3-4 page number of google results and not that beyond and because of this reason any website or blog try to have a god rank on google search result so as to have a good traffic on their websites.
What is the need for Mobile site SEO?
The only need for mobile site SEO is that today many people try to access the site from their smartphones and because of which sites have to customise themselves for mobile web browser too and they have to add certain feature to make it accessible from smartphones too so that theor user can have a good experience on their site.
So they have to add certain features for Mobile site SEO so as to assure themselves they are ranking good.