In certain circumstances the switched off version of a gene is useful. When it is useful because it causes a beneficial modification, it is spread in the population.
For example, the mutations that have changed a brown bear into a white polar bear in the Arctic could be described as advantageous mutations, but the actual basis to the change is loss of genes encoding the production of pigment in the fur.
Advantageous mutations and disadvantageous mutations are, in fact, exactly the same thing – they are both errors that cause dysfunction in the genome. But the effect on the physical body of the switching off of some genes is beneficial, and in these cases they are qualified as advantageous mutations.