Many people consider the role of a music critic, or art critic in general, to be to give their personal opinion on works of art and insult those who do not agree with them. This has lead to a dislike of all critics by some, and the common feeling that the job of a critic is a negative and useless profession. While some critics certainly do fit into this category, it is not the fault of criticism in general, but rather the modern notions of music that are to blame.
At its very base, the role of a critic is simple and important: to keep you from wasting your time and money in some situations, and to encourage you to spend your time and money in others. Music critics listen to the albums that have been made and decide whether they are valuable works that people should buy and then spend time listening to.
This may sound like a presumptuous task, but there needs to be some way of narrowing down the huge number of albums and recordings that are constantly being put forth. How could you have any idea of what to buy otherwise? There are far to many releases to listen to and try out. You don't have the time for that, so the critic does it for you.
That is the tangible job of the critic's music articles and reviews, but there is more to it than that. Music and art are an important, but mysterious area for a lot of people. The general consensus is that music is a subjective product, and that what may be considered good for some people may be bad for others.
There is no doubt that this is true, but that doesn't mean that some music may be objectively better than others, and that the people who call it good are right, and others are wrong. The attitude that music cannot be objectively judged is the reason for the misunderstanding and dislike of critics.
If you thought that music was strictly personal and outside the realms of criteria, it would be disturbing to hear someone try to label and criticize the music you listen to, but it's not the case.
Music exists in the external world independently of our minds and can be evaluated objectively, and just like people can be wrong in their opinions about politics or economics, they can be wrong about music. The job of the critic is to be as objective as possible and steer music towards the objectively good and valuable by encouraging the experience of that music, and discouraging the experience of the rest.
Music and art have the power to be very influential and beneficial aspects of our lives, and just as we should seek the objectively good in other areas, we should seek it in music as well. Critics are just the people we hire to help us do this.