This is so that the image is a better fit for the editor and theme.
The $size
parameter accepts either an array or a string. The supported string
values are 'thumb' or 'thumbnail' for the given thumbnail size or defaults at
128 width and 96 height in pixels. Also supported for the string value is
'medium', 'medium_large' and 'full'. The 'full' isn't actually supported, but any value other
than the supported will result in the content_width size or 500 if that is
not set.
Finally, there is a filter named {@see 'editor_max_image_size'}, that will be called on the calculated array for width and height, respectively. The second parameter will be the value that was in the $size parameter. The returned type for the hook is an array with the width as the first element and the height as the second element.