It is half a year since I have started practicing on a racing track. Here I would like to sum up my experience, mistakes I made and advice I got.
In essence a good line helps to open throttle early and on 100%. You can do it safely if a bike is in up straight position. Also, every racing track is curvy, so the goal is to find straight lines there.
On a straight line you should be going wide i.e. use the whole track width, so you will have more space to turn and therefore carry more cornering speed.