A lot of gore-tex items come with tags suggesting similar treatment. Some of them will have certain caveats ... like dry, but don't iron, or even dry, but pull the item out of the drier as soon as it finishes, so that it doesn't wind up resting on hot, cooling metal.

Anyway, what's going on is that heat re-activates the DWR that came with the jacket. DWR is "durable water repellent," and it's a chemical treatment that acts kind of like static electricity. It's what makes water bead up on the surface, like if you broke an old mercury thermometer. This stuff fades as you brush your jacket against anything, lean back into a chair, wear a backpack, etc. Heat can bring what's left back to life.

You're supposed to treat a parka about once a year with DWR. You can get a bottle for about $10 that will do a dozen parkas. You spray it on ( after the jacket is clean ) then throw it in the drier. On that note, avoid the wash-in DWR, as you really don't want it on the interior of a jacket, getting in the way of your perspiration.