That it was more useful for me to practice easy problems, for beginners, rather than trying to solve difficult ones.
I even recently rushed through ladders problems for 25 kyu players, 15 kyu below my current level. It took about about 5 seconds per problem.
After that, I played a game during which this position was reached. Black (me) to play :
I instinctively thought about A (to increase the vital space) and B (to build a living shape in the corner), but I then recognized a ladder pattern and considered C, that captures the white stone.
I would never have thought of C if I had not taken the time to solve these problems for complete beginners!