This link explains very good the functionality of the drawable and how to support another screen sizes: http://developer.android.com/guide/practices/screens_support.html
Drawable folder is the default folder. I mean, if you don't have the other specific folders (drawable-xhdpi, for example) and your device has this dimension, it will use the default folder.
So, asking to your cuestion, it is recommendable to create a folder for each size(drawable and layout) but you are not obligated (depends on you).