It's about Burst creation...

It's really meaningless to make calcul like you just did.

For exemple you'll use balance card on a monk who has oppened his chakra and got his dmg bonus x3. That way the +10 or +15% bonus damage it would create will be consequent.

The Astro is usefull to creat Burst situations : Enormous protection on a tank with Bole added to his current defensiv abilties to be free to not spam heal on him for exemple, or upgraded balance on a dps who just used is offensiv ability to quickly destroy an add, or MP/Tp card to allow spam.