Typically cylinder lights are used to simulate fluorescent tube lighting
The cylinder light shape simulates light from a cylindrical area source (tube shape).
The Lights page has more detail about the controls. In addition to honoring the standard Maya light attributes, the Attribute Editor will also show the following attributes under the Arnold group:
Cylinder light attributes
Increasing the size of the cylinder light will create a larger area light size and therefore will soften the shadows that run perpendicular to the cylinder’s axis. Cylinder lights will always be circular. It is not possible to scale the width to create an ellipse.