I cared about CR and stuff when I first started DMing; when 5e came out, I cared for...about three weeks.
Then, I went, "You know, I'm going to put stuff where it makes sense. If my players stumble onto something they can't handle, I hope they have the brain to run away."
Most people are expected to use it. First, for players not experienced with 5e, then as people get more experienced and have a better feel, then as a guideline to give a rough idea. Outside of that, the CR is supposed to be used all of the time for AL adventures by everyone. That's why AL adventures have a block at the beginning of each one that says how to adjust encounters based on CR vs party level/composition.