Cheap and a nice solid seal at least for several years. Don’t believe it? Consider this, Champagne aging on the lees is capped with a crown cap until it is disgorged and fitted with a real cork.
It looks inexpensive, because it is, but for many wines it’s all that is necessary, offering a good seal for those table wines you buy, bring home and drink immediately. In fact, one of the biggest drawbacks to the crown cap is that it takes a bit of effort, either a bottle opener or improvisation, to remove the cap from the bottle. Also, once it has been removed from the bottle, it is no longer a particularly useful closure.
Photo courtesy Holly-go-Lightly