Java 8 comes with several new language features.  First, there is support for expressing anonymous functions: Java 8 has lambda expressions and method/constructor references for this purpose. Second, interface methods can have an implementation in Java 8:  there are default methods and static methods in interfaces in addition to the well-known abstract interface methods.
Lambda expressions are essential in conjunction with new JDK abstractions such as Stream, ComputableFuture and the like.  These new JDK APIs are clumsy to use without lambdas and method references.  The non-abstract interface methods were used for the design and implementation of the new JDK APIs. 

Independently of their use in conjunction with the JDK, the new language features may change the way we design and implement our own APIs in the future.  They permit elegant new ways of designing Java APIs and novel implementation techniques in Java.