One of the main reasons for the blunt-appearing rounded nose cone on the earlier builds of the Citation series was for the prevention of ice accumulation. A sharp nose cone is better at building up ice compared to a blunt, rounded nose cone.
In the Cessna Citation series, not building up ice on the nose was important as that ice could break loose and get sucked into one of the rear-mounted jet engines, causing engine damage. In the sleek-nosed Learjet 20 and 30 families, anti-icing fluid is "seeped" around the nose cone to prevent ice accumulation that could damage the engines.
Source: Flying, April 2009. "Triple-A rated Plus" by J. Mac McClellan, p40.