The Ultimaker allows for quick and easy nozzle changes, so different nozzle sizes can be selected to suit each project. Use a smaller nozzle to print fine lines for more detail, or use a larger nozzle to print thicker lines to decrease print duration. The Olsson block brass Jet nozzles have a special tip angle and shoulder width, they also have a special internal geometry with an angle less steep down to the nozzle hole. A special treatment applied inside also makes them slip more.
Note:
This nozzle is compatible with M6 Threads, you can put it on the Ultimaker Original (the fan shroud must be modified) or any of the Ultimaker family that has the Olsson Block mounted.

Clean them efficiently with cold pulls
Print faster, the plastic flows much easier than in classic nozzles
The 0.4mm is the standard size for a lot of 3D printers
The geometry of the inside makes it easier to get rid of plastic so changing colours or filament type is done faster and safer
Use the 0.25mm brass nozzle to print small detailed prints