These are the moments that young players need to show their talent, they need playing time so they can more easily integrate with the core players and understand the philosophy of the coach. Young players are the future of the club, they need to explore their potential so they can enter the main team.
Their presence is very much needed because Munich's schedule is very tight this season, at any time Kompany can make a small rotation or give them confidence in them towards the end of the 10 minutes of the match so that they can hone their mentality and abilities.
Just as you said friendly games are avenues coaches use to test their squad and give playing time to young players especially for the fact that friendly games doesn't have a specific number of substitution, you can substitute as many as you want within the 90 minutes frame. Cup games in its initial stages also brings this opportunity as most times during the preliminary stage lower division teams are fused with the first division teams. So cups brings that opportunity of fielding these young players with 3 or 4 experienced ones to play along side.