Winter is almost here across most of the United States (and Canada...although is it ever not winter in Canada?) and you know what that means...snow, rain, and wind, lots of it in some places. And snow and rain means water looking for a way to get blown in around any hole in your roof or wall, especially round openings for pipes and chimneys.
Every hole that is cut to allow a vent pipe, a chimney, a conduit, or anything that pierces the roof is a potential vulnerability to water. Especially as harsh winter weather blows water all around and over those surfaces, if there is any area not fully sealed, water will find it. The results can be anything from a mild annoyance that is easily fixed with some sealant or foam to a major leakage problem that requires considerable time and/or money to fix.