The requirement for a final else clause is defensive programming. The clause should either take appropriate action, or contain a suitable comment as to why no action is taken.

Noncompliant Code Example

case param
  when 1
    do_something()
  when 2
    do_something_else()
end

Compliant Solution

case param
  when 1
    do_something()
  when 2
    do_something_else()
  else
    handle_error('error_message')
end

See