[eropple]

This is actually half of the value of it. It allows you to easily change the implementation without breaking the public semantics. Generally speaking, the first-pass practice in Scala is to use public fields and to make them vals (final/C# readonly). Where you need to provide mutability, you use a var. But due to C#-esque properties, you can go from

  var x = 1

to (not great code, but illustrative of the point)

  private var _x: Int = 1
  def x: Int =  _x
  def x_= (value: Int) = {
      if (value > 1337)
          throw new ArgumentOutOfRangeException() 
 
      _x = value
  }

The convention is to use empty parents after zero-argument functions that have side effects, much like the ! in Scheme.