Why caulk a roof flashing?

Hi,
What's the point of caulking the roof flashing for the exaust fan vent?
Specifically, I'm talking about what takes place at 2:20 in the following video:

http://youtube.com/watch?v=IrwWOW9RE3Q

It seems to be protection against the water flowing up. Is this done to protect agaist the moisture drawn up by capilary forces? And in that case, are the shingles themselves caulked?
Thanks!
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On Wed, 4 Jun 2008 22:12:40 -0700 (PDT), Aaron Fude

Wind driven water?
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If you have debris build up uphill of the roof vent (e.g. leaves, twigs, snow) it will form a dam and water will accumulate and rise above it.
Ivan Vegvary
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on 6/5/2008 1:12 AM Aaron Fude said the following:

Because you do as much as aesthetically possible to prevent water from entering the house when installing anything that goes through the roof, so you don't have to do it later, before fixing the water damage inside.
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Bill
In Hamptonburgh, NY
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You've seen other replies with good reasons. Another is ice. Ice builds up in places then backs UP the roof and UNDER shingles when it can.
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Hi,
All of the replies make perfect sense to me.
The one thing I'm puzzled by is that all of those would also be good reasons for caulking each individual shingle as well. Yet, that's not the case. Is that becuase there's plywood under the shingles and a hole under the flashing?
Thanks!
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On 6/6/2008 12:05 AM Aaron Fude spake thus:

Well, it's been a long time since I handled any shingles, but I remember the asphalt ones had sticky strips on the bottom near the front edge that would bond with the shingle underneath; does that count? Of course, neither wood shakes nor tiles have any such sealants, and they seem to work fine.
I have a customer with a practically brand-new (OK, 10 years old, but to me that's brand-new) million $-plus house, who reported a leak around a light fixture in a room. Walked the roof, which was in just *perfect* condition; nothing loose, not a crack where one could see a chance for water getting in. Turned out it happened during a storm this spring where high winds drove rain under the shingles. In a case like that, there's not much that's going to stop *some* water from getting in somehere.
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On Fri, 06 Jun 2008 01:19:28 -0700, David Nebenzahl

On a tile roof the tile only protects the membrane under it. The membrane is the water proof part.
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That looks like a fairly flat roof they are installing on, so my guess is that they are trying to prevent 'run-under' where the water isn't necessarily backup UP under the shingles, but running in from the side.
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Mark wrote:

Remember also that strong winds can blow water all over the place and even into tiny holes or cracks.
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