The harvest is a crucial moment for the wine production where the producers lookk for the best balance between several components in the grapes: sugars, acids and so on. There is not a standard rule, it also depends on the wine style the producer is going to make as well as the weather conditions that will influence the plans!
In case of Late Harvest, the grapes are collected later in order to concentrat sugars and flavours and to reduce acidity.
It is easy to image that this practice is often use to produce sweet wines and it allows to still get a good freshness compeared to sweet wines obtained by grapes dried after they have been picked. It is not very coomon in Italy while it is in other regions such as in France where the “vendange tardive” is appreciated.
Anyway, this practice is also used to produce dry wines which will be more intense.