buy just one share to 'watch' or monitor the stock. I may not want to invest $5M in a stock I'm unsure of at the moment, so I might buy just one...and see what the stock does... if it continues to go UP... then I may buy more. If it's going down, then I don't lose much.
If I short just 1 share.. the same principal applies.. it the price is rising, I don't lose much $$... and I may decide to buy it long. If the price continues to drop, then maybe I'll short more shares...
It's a way to monitor a stock, and yet limit how much you may lose if the price goes the opposite way of what you want... or it may just be that I want to watch what that stock is doing...