The outbrief has been a bread and butter part of the flying process for me since I joined back in 83 and continues through to today. I have been involved in supervising as well as leading and flying in general and imho the outbrief forms a vital part of the assurance process that everyone flying has all the relevant information and covered all the necessary points.
A brief is fine as far as it goes but there may be other elements not directly part of your sortie brief that you need to know and the outbrief process covers this. It is a catch-all, usually not needed as the information has been covered but it regularly catches something or things that haven't been covered and is therefore, again imho, a vital part of the process of going flying.