As stated, shade is one reason.
But actually it is, IMHO, somewhat like a third arm. If set up and utilized properly, it serves multiple purposes. It is an extension point for lead and ground, as well as electrical for grinding. It can also be rigged to hang grinders from, thus keeping them clean as well as safe and eliminating a portion of the many trip hazards involved with pipelining. I have seen some that were set up with a "rigged" army tent that worked great for an extendable as well as mobile welding "hutch".
There are many different styles of "booms" and I would be interested in viewing photo's of "multi-purpose booms".
It is really interesting to see some of the ingenious ways that pipeliners utilize this extension of their rig.
jrw