The GEN lamp supplies a very small initial current to the alternator rotor to enable the alternator output & charging process to begin as soon as modest rpm is had.  If the lamp burns out, the alternator may not provide any output. This article describes the system and a popular modification which eliminates any lack of charging from lamp burnout (which is actually fairly rare). 

General comments about ‘modifications’:

As a general rule, ‘The factory knows best’ is a fairly accurate statement. However, few of us have UNmodified motorcycles. Our bikes are a reflection of our personal statements and desires; and, are thus often modified for many reasons. I am NOT in favor of many of the modifications that we all see. Some of these modifications, which I have done myself, are not very economical…or…do less than is often believed. Some modifications can REDUCE reliability.