SpriteFont is an image. It will always look exactly the same on all devices. While Text object may be rendered differently, depending on the browser, installed fonts etc. There are lots of other differences - SpriteFont can be scaled smoothly, you can control the spacing for each individual character etc. And of course, since it's an image, you can add any effects to characters, outline, shadow, or even replace characters with pictures.
For example, you can display a message "You collected 10 #", where instead of # symbol there will be a picture of a golden coin.