First, easiest way, it is a "learning tool" because it can show you if you are bad or no. if you need to improve a lot or no.
Silly to say but we need this to know. lets say, tomorrow you become a lumberjack, you cut trees, you cut 10 trees a day. knowing this, are you able to say me if it is a good or bad number? now i say you that your neighbor does 20 a day... now you can answer.
Second, have to learn to read it. we know that 3 > 1 because teachers said it. parser can list all skill you used. How much you used each, how much time you did have crit for each skill.
So you did a rotation for the first try, you know there is matter because at 1 minute 15 second, the boss go away, you cant anymore damages. but there is maybe things to change a little. lets do a out of GCD skill a little sooner than expected maybe it will allow you to get it a little more before the boss jump, so a little loss on one point to a little win after. try, and lets see what parser say, what from the 2 thing does the most damages (and remember to see how much critical etc)
Third : you are in a team. you are a samurai ok. but, when the RDM does his buff, it improves your damages. while watching the parse of the whole team, you think the RDM could delay 5 or 10 second the buff to get more damages. It is not so stupid sometime. (a skill with 2 min cooldown in a 10 min fight can be use 5 time. you can delay some of the uses to get it 5 times)
Oh, and, with the graph you see that at a time, the 2 healers clearly drop in damages => why? maybe tank can change a little to get this drop shorter? or even DPS?
There is example because i dont know how to explain well. but the main matter is to learn to well read it, not just "hey i did 5% more damages than last time"
Doing the rotation perfectly? yes good, when the boss dont move, when you dont have to move, etc. during each fight there is plenty thing to adjust, and not all time stuck to exactly the rotation...