Rain is one of asphalt's worst enemies.  If the asphalt has not been sealed by new construction, or some sort of pavement preservation technique such as chip seal , there are likely cracks in the roadway.  As water penetrates the asphalt, water enters the base material under the asphalt.  This weakens the asphalt and reduces the load capacity of the underlayer.  This causes depressions in the asphalt.  As traffic drives over the weakened area, the asphalt begins to crack and a pothole is formed.  This continues until the maintenance crews can get out to repair it.

 Maintenance crews are not likely to go out and repair these areas when it is raining. If a pothole needs to be fixed when the roadway is still wet, they can use the cold patch but it isn't a permanent fix. 

Chip sealing is a pavement preservation technique that extend the life of the roadway.  It has been found that it is more cost effective to to do the pavement preservation techniques than to have to repave.  Crack sealing is also a preservation technique that can help extends the life of the road and prevent potholes from occurring. This is a proactive approach to sealing the asphalt before water can create problems.