oEmbed is a powerful WordPress feature that allows you to embed other website’s content simply by pasting its URL. WordPress allows you to oEmbed videos inside your blog posts without using any additional plugin, you can extent oEmbed functionality to Twitter Tweets by using oEmbed Tweets plugin.
WordPress default visual editor provides you a minimum set of formatting buttons in visual editor and for advanced formatting of content you have to use HTML editor. You can make formatting easy and fast by adding more formatting options (like options you have in office suites) inside visual editor.
Default WordPress text widget accepts HTML but for them who faces problem in designing text in HTML have an easy option of using a widget that displays visual editor.