Into a canned DRAUGHT Guinness I found a plastic white sphere. What is its purpose?
It's called a floating widget and is used (together with nitrogen instead of carbondioxide) to give the beer it's typical, creamy head when poured from the can. The method was invented by the Guinness Brewery in the 1960ies.
Cite from Wikipedia:
Some canned beers are pressurized by adding liquid nitrogen, which vaporises and expands in volume after the can is sealed, forcing gas and beer into the widget's hollow interior through a tiny hole - the less beer the better for subsequent head quality.
When the can is opened, the pressure in the can quickly drops, causing the pressurised gas and beer inside the widget to jet out from the hole. This agitation on the surrounding beer causes a chain reaction of bubble formation throughout the beer. The result, when the can is then poured out, is a surging mixture in the glass of very small gas bubbles and liquid.