Isis wanted a strong base from which to develop and multiply its influence. A war-torn country with established discord between different religious groups and with many Muslims provided an excellent opportunity. The civil war diverted attention from the activities ISIS was carrying out. Then Assad used ISIS and Al-Quaeda to undermine government opposition groups and to seize territory. Assad is responsible for the power ISIS weilded.
Their existence made Assad's irreversible victory in the country's 8-year civil war possible. Having allowed distinctions between ISIS and other terrorist groups to blur, Syrian Arab Republic under Assad was promoted internationally to represent legitimate government, while any dissenting group was lumped with ISIL and al-Qaeda as dangerous terrorists. Western nations have now accepted Assad as the lesser of two evils.
ISIS originated out of the US invasion of Iraq in 2003. Al Qaeda chose to capitalize on the Sunnis, and they established al Qaeda in Iraq to wage an insurgency against US troops in Iraq. During this time, they were active in waging a factional war against Iran backed Shiite militias in central Iraq and bombing hotels in neighboring Jordon. Many of their members were incarcerated in US-run Camp Bucca where they were able to meet up and radicalize.
During the Iraq war, AQI would go back and forth between Syria and Iraq to resupply. When Assad began his reign of terror, peaceful uprising turned into civil war. AQI saw an opportunity to establish a presence there. It quickly moved into Syria. As the Syrian civil war continued, ISIS became the first rebel group to capture major cities.