Racing lines are good to learn a new track if you are struggling. However, only use it for a lap or two to "see where the track goes."
You don't want to get "good" at using the racing lines because it causes bad habits:
1) you don't actually learn the track. A big part of learning any new circuit is picking up visual cues around the track for braking points, turn-in, apex, and track out reference points. If you are looking at the line, you don't ever learn these as well.
2) Looking ahead. This is the biggest mistake of beginner drivers in real life, and it absolutely translates to the track. If you are always looking at the line, you have a tendency to start looking only 1 reference point in front of you (i.e. if you are approaching a corner, you have the tendency to stare at the turn in point). Really as you are approaching you should be locating the apex, if possible, or even the track out if you can see through the corner.