Also

Single-expression functions are very nice and concise:

fun multiply(a: Int, b: Int): Int = a * b

But often, you have a single-statement function, that also needs to write to a log, for example.

You could write it the following way:

fun multiply(a: Int, b: Int): Int {
val c = a * b
println(c)
return c
}

But then it's not a single statement function anymore, right? 

And we also introduced another variable. To the rescue, also():

fun multiply(a: Int, b: Int): Int = (a * b).also { println(it) }

This function will set results of the expression to it and return the result of the expression.

This is also useful when you want to have a side effect on a chain of calls:

val l = (1..100).toList()

l.filter{ it % 2 == 0 }
.also { println(it) } // Prints, but doesn't change anything
.map { it * it }