The assumed purpose for the terrorist organization’s invasion is to seize Iraq as part of the Caliphate the terrorists claim to be restoring. This may be true. However, there are other purposes that may be in play, also, if not instead.
One such purpose would be to destabilize Iraq while exposing the paper tiger that President Barack Obama’s United States has become. This purpose envisions ISIS shooting the place up, exacerbating along the way extant sectarian violence, and then withdrawing into Syria, if not into strongholds in northwestern Iraq. Keeping captured weapons, while demonstrating ISIS’ reach, to their economic betterment from…supporters…of the drive for the Caliphate.
And there’s this:
[ISIS has] obtained significant numbers of tanks, trucks, and US-origin Humvees in recent military operations in Iraq and those arms are being shipped to al Qaeda rebels in Syria[.]
Now one branch of Syrian rebels has several examples of our weaponry.
Oh, and this, admittedly a bit of a stretch as an ISIS purpose: Iranian forces are now operating in Iraq. The enemy of my enemy bit.