Simpleton Syntax Question

Ok - So ive been moving onto AS3. And during the change over from AS2, I have begun to notice that as a standard practice in writing classes etc, that people are coding with an awful lot of use of the underscore … ie… var _myx:Number = 5; private var _children:Array; etc …
Is there an explainable reason for why people are adopting this way of writing? As of presently, it makes me feel sick looking at, as if ive missed something out.

thanks for any bulb to aid the dark