Think shingle stagger is not really important? Well here is s photo of what an improper roof stagger can do. As you can see in the photo you have staggers of 2-3 inches between the different courses of shingles. What you have to imagine is water running down the roof and getting in between the two shingles where they meet together.
We were called to shovel snow off this roof becuase it leaked when there was snow on the roof. Thought ice dams were the main culprit to roof leak.
In the spring 10 squares of shingle were replaced and found the shingles yo be improperly staggered causing the roof to leak while there was snow. In a rain the leak was too minor to really amount to significant water in and he building to ever be noticed; however, while snow was melting for days on the roof it allowed for insulation to become saturated and enter the building in buckets.
So the leak had the appearance of snow and ice dam causing the leaks but actually the leak was cuased by improper shingle stagger.