There are several reasons for a method not to have a method body:

Noncompliant Code Example

public function doSomething() {
}

public function doSomethingElse() {
}

Compliant Solution

public function doSomething() {
  // Do nothing because of X and Y.
}

public function doSomethingElse() {
  throw new UnsupportedOperationException();
}

Exceptions

Empty methods in abstract classes

abstract class Animal {
  public function speak() {}  // default implementation ignored
}