I think too many people are confusing raid logs with parsing.
As a few people have pointed out, a raid log is a tool that measures the performance of a raid. It can be used by the raid leader and raid members to see what they died of, who didn’t un-curse or dispel, or which hunter just basically auto-shot their way through the raid. If a boss has a DPS check associated with it, the raid can use the log to determine who was lacking in either gear, consumables, or general class knowledge to help them get to that DPS threshold. A raid log is an important and viable tool to help a guild or raid group successfully navigate a raid.
Parsing is, as others have mentioned, simply a comparison of all those raid logs. Parsing really only exists so that people can measure their e-peens against one another and as has also been mentioned here by others can be VERY misleading with respects to how well a class is played. A healer who doesn’t parse high on their heals might have done so because they were busy dispelling magic effects from raid members while other healers simply kept healing. A damage dealer that doesn’t parse well could also be contributing to the mechanics of the raid while other damage dealers are padding their numbers by ignoring what is good for the raid and continuing to deal damage for higher numbers.
A raid log is a very good tool and can help a raid be successful with raid challenges.
Parsing from those raid logs is simply a means for people to show and compare their e-peens and often times is not a true reflection of class knowledge or performance.
I hope this helps. 
Thanks.