TinyMCE is WYSIWYG (What You See Is What You Get), which means that any formatting changes you make, like making the text bold, are immediately reflected in the editor. This gives you a better idea of how your posts will look when they are published. TinyMCE is used both by the Visual Editor when editing posts and pages, and as part of the Text widget editor. The non-TinyMCE based post editor in WordPress is called the Text Editor, and it is not WYSIWYG.
The upcoming post editing experience in WordPress, Gutenberg, is WYSIWYG, but it does not use TinyMCE. It is meant to replace both the Text Editor and the Visual Editor when it is released together with WordPress 5.0 in the near future. WordPress users who want to retain the old post editing experience, with the TinyMCE based Visual Editor, can install the Classic Editor plugin.
The look and feel of TinyMCE in WordPress can be configured by editor styles, which are included in WordPress themes to make the TinyMCE editor look more similar to how the theme presents post content. The formatting options can also be extended using TinyMCE plugins.