from these 15 bonus point you could have bought some high attributes because this is one of the most efficient ways to spend them. If you want to buy some nonfavored attributes buy them , or at least the first dot with the 10 points that you have for boons and attributes.
For example if you want to have charisma(non favored) and stamina(favored) both with 3 epics.
Buying them with exp alone would cost you 45 exp. but if you use 3 dots , for example to gain stamina 3, it would cost you 25exp, but, if you instead used those 3 dots to buy charisma you would only have to pay 20 exp, and if you would have bought charisma 2 and stamina 1 it would be 22 exp
Remember that you gain another 9 attribute points (in a 4/3/2 spread)when going from hero to demigod that can't raise you attributes to 6 so plan accordingly so that you can use them while upgrading. and most of those 10 additional points for boons and attributes that you obtain when transferring to demigod where used, at least for me, to increase my Epics from 3 to 5 because prior you are unable to do based on the legend score.
and remember you ony need to pay curent rang x4 for a boon if you buy them in order in a purviev