This WordPress User Guide Media blog post will help you understand how the Media manager in WordPress works. While the media manager allows you to upload your media into an organised folder structure on your web server. When images are uploaded they are converted into smaller images so you can embed them into your web pages taking up a smaller footprint. Having smaller images on your website is very good for SEO because it will load faster. Having a faster website is good for your users experience and will help with your search rankings. Understanding this media uploader will benefit your website.
WordPress User Guide Media