Don't let anyone tell you it's "just" for Parcoursup, that's wrong. Continuous assessment really does count toward your final bac grade - it's 30% like you've heard. But where it gets a bit confusing is that this same grade also goes into your Parcoursup application. So yeah, it counts twice in your life as a student, but for two different things. One is your diploma, the other is access to higher education. You can't really ignore one for the sake of the other because they both follow you.
Honestly though, I know 30% might seem small compared to the final exam which is worth 70%, but zeros don't go away. If you end up with weak continuous assessment grades, even a good performance on bac day won't fully make up for it. They work together. Are there particular subjects where you find it especially hard to keep your grades up, or are you just wondering if you need to be this rigorous everywhere? Because stress is never a good strategy, but consistency - that is.