Corch,

Sealing the hingeline prevents air from flowing through the gap.  This is especially important when you're pushing or pulling as the increased angle of attack can force a lot of air pressure there.  If the gaps on both sides are unequal( and they likely are) you can get some induced roll when you push or pull that you can't really trim out.  

Granted...in this day of paper CA hinges, the gaps are very very small and this really may be a relic of a day gone by (older pinned hinges tended to create a larger gap and this was more critical), but for those of us that have been doing this a long time, it's habit.  <br type="_moz"/>