If Sato goes quick he will place higher in pre qualifying. This means he starts in reverse order in qualifying. Button took pole, so he'll be last man out in qualifying. Now if the rain holds off then Sato may get a quick lap in with dry tires and people like Alonso may have to use rain tires thus at a disadvantage. The teams foresee rain coming in session 2, so they slowed the guys down to take advantage of current dry conditions and run in dry weather tires and qualify faster and when the rain comes the other guys will have to use wets and thus going slower.