That is a really good question. Java 8 has lambdas and streams, and Java 9 has jshell. So why bother with Scala? I give you three reasons.
1) If you use something like Play or Spark, you'll find the Scala API more powerful and convenient. Run and learn Scala.
2) In Scala, the functional features aren't obscured by backwards compatibility
Java has lambda expressions, but there are no types for them. A lambda turns into a functional interface, and that makes it really awkward to express certain
patterns that should be natural. I found it easy to understand and use lambdas because I know how to use, compose, and generate functions in Scala. My students who learn lambdas have a significantly harder time with functional styles in Java because they don't. So, you may become a better Java programmer by knowing the basics of Scala.
3) Scala shows you what you can do beyond Java
Java 8 is nice, and Kotlin is a bit nicer (more regular, less boilerplate), but they don't expand your mind a lot. Scala teaches you a whole slew of new techniques, particularly when you get to the more advanced topics, that give you a glimpse of what the next big thing in mainstream programming languages might be.
Cheers,
Cay