In addition to fuel, your motorcycle has a number of liquids on board for specific tasks, like cooling the engine or improving its performance.
Although we often speak of a "water-cooled engine", it isn't cooled with water at all. Modern coolants are of a more complex composition. Apart from carrying the heat from the engine to the radiator, this fluid also prevents oxidation in the engine block and contains an antifreeze.
Additives are added to improve fuel quality, increase performance or prevent problems in the longer term.
For example, using an additive at your last fill-up before a winter break keeps your petrol in good condition. Modern fuels contain ethanol, which has a few nasty properties, like extracting water from the air.