Lines are just that, a Line. There is nothing special about a line, but a Polyline has many advantages as described below.
Polylines are a series of lines that are joined to each other to create one long line. There are advantages to using Polylines as listed below:
It is easier to manipulate a Polyline rather than multiple single Lines
You can change the Width (thickness) of a Polyline
You can colour fill a closed loop Polyline
You can convert Polyines into other objects such as Stands, Zones, Gangway Carpets and Hall Boundaries/Gross Areas
They can be converted to Splines