ISI defends from all enemies, external and internal. You dont know what it does and how it does so dont worry.
my dear brother ... that was very short. ISI has a way way bigger role in the whole of world than anyone can digest. All the UN assigned missions to Pak Army are maintained, planned, executed and put into action by ISI ... be it Congo, Somalia, Nigeria, Niger, Algeria, Sudan, Bhutan, Sri Lanka, Chechnya, Bosnia ... but you will notice that all these nations are very very poor. So ISI is literally doing a welfare service to humanity.
On another note, ISI is also sensible enough to side with poor, oppressed, victimized and such ... for instance, RAW sided with KGB in 1979 ... and we all know Soviet Union proved to be a sinking ship. Now again, RAW has sided with CIA in Afghanistan ... and it is proving to be a sinking ship again. It is another thing, that when a ship sinks ... rats are the first ones to abandon it. Such is the case everywhere ... regarding RAW, CIA, KGB, Moussad.
ISI is the only agency that has shoved anti-state terror back to its mother's womb in 1980s ... and indian mourners are still crying through MQM for that para-military operation. So you see, ISI has long lasting effects and its role is universal ... even US media values ISI's stance on things happening in US. Remember they interviewed Gen. Hamid Gull over 9/11 fiasco ... and agreed that it was an inside job. And it was ISI again who told india that silicone valley will blow up in smoky clouds and those war-mongering mental cases stuck their tails within their legs and bolted for their holes.