I will sum it up.
If you, in this case, press the Thunder Clap ability, we have removed the effect, damage and slow et c, a dummy will be summoned, via triggers. You can order the unit to cast a spell for you, I.E Blood lust, so you don't have to target yourself with it.
Dummy units are very good for spell making, they are actually one of the key parts.
Short:
They can do what spells in object editor normally can't.