Dirt clogs the pores.  Cleaning it opens the pores up.  Just like a teenager's pimply face!

They're also coated with Durable Water Repellant and will occasionally (annually/bi-annually) want to be re-done.  A run through the dryer can re-activate the DWR, basically melt it and let it re-surface the jacket.  Nikwash/Nikwax products do a decent job.  I've heard some folks say a careful ironing can improve the performance after the jacket comes out of the dryer.

How do these things let water out, but not in?  Trade secret.  I suspect it has something to do with the size of the pores & the relative size of the water being blocked or vented.  Vented water is vapour dissolved in the air.  Blocked water is liquid.  The heat from your body creates a high pressure zone inside the jacket and pushes the water vapour out.
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