Confidence in a job well done
While you may certainly be able to do a superb job yourself, what if you can't? Will you know if you got it wrong? Have you studied the code? A licensed plumber in your area has. That same plumber should also be able to provide quality work.
Some jobs require a permit
Can you pull a plumbing permit? Unless you are a licensed plumber in your area, the answer to that will be no. Did you know that it's a violation to do some plumbing without a permit? To avoid violations, it's always best to work with a licensed expert plumber for anything but very minor repairs.
What if you make the problem worse?
If you are not a plumbing expert yourself, there's always the chance that you may make matters worse. That small leak in the bathroom that you thought you fixed with a simple tightening of the pipes might have a bigger problem lurking in areas you can't see. Only an expert plumber will know how to properly and completely resolve certain plumbing issues.
If you are confident that you know what you are doing and can get the necessary permits required, go right ahead. It takes a strong person to admit when they are not the right one for the job.