You need to roflstomp the opponent before in your own league before CL kicks in. People might laugh about Bayern trashing HSV and the likes 9-2, 8-0 or so but there's a reason
First of all is it the mentality just to attack, even if you're infront 5-0. Might seems like disrespect to the opponent but it's actually the opposite as it shows you're taking the game serious no matter who you play
Second is it because it makes it way easier when you meet the best opponents that your attack and team is firing on all cylinders. It all depends on which country and what style of play but I don't believe in this "conserving energy" thing, I think it's rubbish in todays football. It might have worked well 15-20 years ago, but football develops all the time
Tiki taka was the shit in the late 00's/start 10's, now you don't win anything that way