Second, during the fermentation process the bottles are stacked upside down - in order for the solid matter that results from the fermentation to come to rest on the underside of the cork (it is later expelled and the bottle topped up during the degorgement process).The indent allows the bottles to be safely stacked with the neck of one resting on the indent of the bottle below.In wine which throws a sediment, the indent means that the amount of wine you have to leave at the end of the bottle is less than if the bottle had a flat bottom.This is why more expensive wine tends to have the indent more often than the cheap stuff which doesn't have sediments.Wine bottles used to be individually blown and hence were spherical, somewhat like Chianti bottles.The more stable, straight-sided bottles were produced with the aid of an iron rod pressed into the bottom of the bottle while the glass was still workable, leaving this indentation.
Most cheaper wines are filtered or whirled in a centrifuge before bottling, avoiding sediment and thus the need for the more expensive punt-bottomed bottles.
One reason I heard was that it provides a place for light to "sparkle" - and therefore show off the colour of the wine nicely.
And while all sparkling wines (as far as I know) have indents, not all still wines (even some fairly decent ones) do - although most do.
I suspect that the punt is another of those wine snobberies, like the ridiculous practice of stuffing the neck with a piece of bark, which tends to taint the wine and is no cheaper than the much more efficient screw cap.
And if someone writes to say that corks "breathe", I would like to see them prove it.
Many wines continue to develop in the bottle and this process produces sediments.