I get your frustration with preseason games especially when new coach like Erik ten Hag starts at Bayer Leverkusen. It is hard to see team lose badly even in practice matches and it does not make fans feel good about season ahead. While these games are usually just for getting players fit and trying new strategies so early losses can seem like bad sign especially after big coaching change. No one wants their favorite team to lose and it is normal to want them to start strong especially when new exciting players have joined. Hope that these new players will make difference is what keeps fans optimistic during tough times.
This preseason games also gives the coach to know what strategy is best for him to use when the league begins. When you try out a mixture of some players and they don’t fit well for the pattern you want to play, you have the preseason games to try them out and be sure that they work well for you. Eric Ten Hag can also try playing one or two games with bigger teams to prove the quality of his team and to know the strategy that will best work for him. Bayern Leverkusen are a top team that many are looking up to make the next season in the Bundesliga very interesting and they need to make sure that they’re a challenging team to Bayern Munich, only with then can the next season begins.
You're right this preseason is the best time to try all tactics and know which one that will work best in the team most people don't take preseason games too serious but that should not be the case because preseasons is a testing ground for any team and correct any area that is not working effectively because when the season starts properly there is no time to do experiments as doing that is at your own risk