Drawing in Flash is messy--especially with a mouse. Your lines wobble, your shapes squiggle, and everything's just a mess. There are settings on some of the drawing tools that will smooth them as you go--but if you've already spent ages working on a drawing and want to just smooth it out without having to redraw it, that's just too much trouble.
Flash does have tools to help. You can select an object and then use two little nifty tools that are part of the options for the Arrow Tool: the Smooth tool (left), and the Straighten tool (right). They...well, they do pretty much what you'd think, but let's demonstrate a little, using the quickly-scribbled squiggle above.