IS-IS is a link-state routing protocol that is similar in its fundamentals of operation to OSPF. Your 3850 supports IS-IS just like it supports OSPF, EIGRP, or RIP. Whether the IS-IS is helpful to you depends on your needs. You should first ask yourself: "Do I need a routing protocol between my two 3850s?" If you need a routing protocol then IS-IS can be considered as one of the prospective protocols to go with.
I had a webinar regarding the fundamentals of IS-IS here - perhaps it will be helpful. Look for the recording of the "The Return of IS-IS".