A simple example.

A four sided box drawn with the line command consists of four separate entities.  A four sided box drawn with a continuous polyline is a single entity.  If you have to move, copy, or rotate the box it would be simpler if it were drawn with a polyline.  Polylines can also be given a "width" which is something that cannot be done with a line.

While splines may be useful to use many people can go their entire lives without ever having need to use them.

What use are polylines?

Do you ever hatch irregular shapes?  How do you do it?  Do you select an empty area somewhere in the middle?  Will that always work if it contains additional geometry?  Do you select all of the individual line segments?  That must take a lot of time.

Think polyline.  Hatch.  Select object.  Done.  It doesn't all have to be on the screen.

Do you ever need to determine the area of something?  How do you do it? 

Polyline again.  Just select it and look at Properties.

Do you ever want to draw an object with real width?  How do you do it?  Two lines maybe separated by the width?  Then filled in?  Too much work.

Polyline again.

The list goes on.  Personally (in 2014 at least) I like the ability of adding or removing vertexes.  You can't do that with a simple line object.

Dave

In addition to things already mentioned....