Why is there a floater in a Guinness draught can?

yes.

The little plastic ball called a "Widget" was first patented by the Guinness company years ago to add the right kind of head on their thick and creamy stout, which is less fizzy than regular lager beers like Bud Light.

When Guinness is canned the brewers add a shot of liquid nitrogen to help pressurize the can.

As the pressure in the can rises, compressed nitrogen and beer are forced into the sphere through a tiny pin sized hole.

When you open the can, the beer and compressed nitrogen within the sphere depressurize and are shot out into the beer. This disturbance causes the CO2 to bubble up which creates the foamy head at the top of the glass. The only thing left to do next is drink!