The first open tournament I ever won was due to my knowledge of the Lucena position.
In the last round, there were four of us, a master, an expert, and two A players fighting for first place. Incredibly, both A players reached won R + P endgames against their higher rated opponents. I knew the Lucena position and coasted to an easy win. The other A player didn't, and only drew.
As a result of that difference in knowledge, I won clear first. I also leapfrogged from 1899 to 2001 in the ratings. I never hit the 1900s until many years later when poor health affected my results.
That event reified the importance of endgames for me. I realise it also shows that one can become an A player without knowing the Lucena position. But life is much sweeter for the chess player who invests some time to learn the basic endgames.
How do yo explain opening principles (center control, development, castling) with a series of endgame positions?