It is a little more than being the first.
A couture gown (only 10 houses can claim the name) is custom made for the lady who may have exclusive rights for that season.

You are right that after being shown, there are companies busy producing knock-offs, and copies that are mostly like the original, and then manufacturers that make a dress that doesn't cost as much in fabric, time and trouble to make, internal engineering, and finally it gets watered down to something that might resemble the original in a good light and is available in a year or so at a discount store.