I remember being asked this many years ago – in the days when as often as not I used to take the question master's word for such factoids and write them down with barely a second thought. Then I heard nothing more about it, until December 2018 when it came up in Stockport Quiz League.
So I thought it was about time I checked this out. Naturally I turned first to Wikipedia, where I discovered that no one knows why the dimple is there. Wikipedia has eleven possible explanations, many of them involving added strength or more efficient storage.
Wikipedia also tells us that the dimple is sometimes known as a kick–up. I may be wrong, but this seems to me to have an antipodean ring to it.
© Haydn Thompson 2020