Good question ...
The motive was so to avoid having the use dattime comparisisons to identify which version of an entity we used in a decsison making proccess, instead just lay down a single version identifier ....
So instead of tracking decision X based on Entity 1 was made at 01-01-2010, we'd track that decision X was made based on Version Y of Entity 1 at 01-01-2010.
This way i could avoid the date time compares to figure out which version of Entity 1 was in effect when the decision was made, since we'd have an identifer to look up.