Not really needed, but it's a good way to cover recessed areas that you want to have more shadow (or you accidentally painted with a bright color).It's also a good way of toning down a color (say, you want a more muted red: wash it with devlan mud or gryphonne sepia). Also note that using washes doesn't mean you'll save time automatically--I use washes but still spend an hour painting faces.
For example:
That took me 1.5 hours to paint the face. Around 2 three hours on the cloak. Thing is, after I washed it I would re-highlight stuff and wet blend stuff and... well you get the idea.