my son from my testing wile playing it doesn't seem to do anything other than free space on the promotion screen. but my judgement may be clouded from that lack of intricate mod knowledge

It gives XP to the unit type being rewarded, though the XP is usually negligible or discarded as the ones most likely to qualify for promotion are the top tier units who have no use for XP

thanks guys^^

The cost also doesn't seem to be deduced from your money, so it's really just as Emperor of Mankind says - it frees up the promotion screen and nothing more.