What does a stand alone "T" index value do?
T stands for "Tool" and has its origin in the origins of .gcode being for other automated machine controls. Depending on the machine, everything could be a tool for .gcode, like an actuator or pump or a spindle motor or a drill.

In 3D printers, the T-controlled tool is usually the extruder motor. Convention has it that the indexing always starts at 0, so T0 and T1 are your first and second Extruder respectively. It is the way to choose which tool is used. 

As a side note: E is not originally intended for extruders but for the feed rate of lathes.