The raster copy trades infinite scaling for easy placement
EMF can describe drawing commands that scale with a page. PNG stores a fixed pixel grid. The PNG is convenient for software that expects an image, but zooming or enlarging beyond its chosen dimensions can reveal jagged edges and softened text.
Keep the EMF for future exports at different sizes. If the graphic will appear in several contexts, produce each needed resolution from the source rather than stretching one small raster output.