The term caliphate is commonly associated with Islam and is the title of the leader in the Islam religion. The caliphate is actually the one who leads after the founder of Islam, Muhammad. ISIS says that they fight and if they die then they will be rewarded after they die. The first caliphate occurred in 632 in Arabia near the peninsula. The first leader known as the caliphate was Abu Bakr as-Siddiq. This took place after the Prophet died.
Since ISIS has become associated with using the word Caliphate to describe their leadership, this associates them with the Muslim religion. This enables them to be associated with Islam and that they lead Islam due to being a caliphate. The caliph is a very important title.