As wages and salaries rise in nominal terms under the influence of inflation they become more highly taxed, even though in real terms the value of the wages and salaries has not increased at all.
Now suppose that due to inflation, their wage goes up by 5%, but the government does not increase the tax threshold.
Thus the proportion of the person's income that is paid as tax has increased.
Since the wage increase was due to inflation, the person's net real income declined.
Many voters do not perceive the effects of bracket creep, and so the government may prefer to adjust tax brackets manually once every few years: in effect, restoring the real tax rates to their approximate pre-inflation levels but in a way that gives the government the appearance that they are cutting taxes.