If you don't understand the purpose of this attack, look at the effects:
* Keeping the populace fearful and terrified and "clamorous to be led to safety"
* Increasing police state power
* Legislation will be passed making it easy for RV owners to be searched and harassed, when previously they enjoyed some level of legal immunity
* etc
The purpose is only unclear if you assume it was just some "lone nutcase" who did it, according to the official story, rather than what it actually is: yet another a false flag attack, to push a political agenda.