Why is grounding required in a transistor circuit?
Also, what is the use of defining a voltage reference point? 


All voltages are relative, a concept which a lot of people have trouble understanding. 

It is rather common to say that "at Point A we have X Volts" but actually it is wrong!

Officially you should say "Between point B and point A we have X volts".
As that become tedious it is easier (common) to define a reference point. Then if you say "at Point A we have X Volts" your are actually saying "Between the reference point and point A we have X volts".

The next step often used in a schematic when defining the reference point is to use the ground symbol for that.