Joomla uses a specific folder for images used in Articles and other content items. The folder is here:
The images folder is where content images start. User created folders can be nested within the images folder. The level above contains system images that should not be deleted.
When uploading an image using a third party text editor such as JCE the images folder becomes the upload root. But you can set another images root in JCE if you want to keep your images elsewhere.
If you have many images to upload it's faster to FTP to the images folder than to use the Media Manager.
The images folder is also used by third party extensions to store media such as audio and video files.
You might wonder why the images folder isn't named articles. Many third party extensions depend on this naming and would fail if the name was changed.