Before that (e.g. on MacBook Pro) it was very hard to see the microscopic selection-circle hidden behind much bigger cursor arrow when trying to snap e.g. a line to some vertex. Now this setting can be used for such displays to increase the marker size and make it clearly visible behind the cursor.
The already existing "sketcher marker size" property is intended to be used for sketcher only, so it's not suitable for global setup of all the markers' sizes uniformly...