A long time ago someone asked me to do one, and though I didn't know what one was, I did it anyway. Some excellent artists make it a point to never do demos, and any amount of love or money won't tempt them. Others seem to make their living in demoland. I find I can handle about five a year. The church halls and the hills are alive with painting clubs with social, economic and educational reasons to foster demos.
Many members are late to the post and appreciate the artists who take the time.
Some demo-doers are wizards. Horizons are widened, techniques are picked up, insider information is oozed by osmosis, and it's satisfying and perhaps empowering to see a long putt well sunk. I've never failed to learn something by attending somebody else's demo. We are all specialists. At the same time the giving of demos is a learning process.