Hans-Hermann Hoppe very clearly explains in this article. Below is an excerpt:
…what explains Hayek’s greater prominence is Hayek’s work, mostly in the second half of his professional life, in the field of political philosophy — and here, in this field, the difference between Hayek and Mises is striking indeed.
My thesis is essentially the same one also advanced by my friend Ralph Raico: Hayek is not a classical liberal at all, or a “Radikalliberaler” as the NZZ, as usual clueless, has just recently referred to him. Hayek is actually a moderate social democrat, and since we live in the age of social democracy, this makes him a “respectable” and “responsible” scholar. Hayek, as you may recall, dedicated his Road to Serfdom to “the socialists in all parties.” And the socialists in all parties now pay him back in using Hayek to present themselves as “liberals.”
Now to the proof, and I rely for this mostly on the Constitution of Liberty, and his three volumeLaw, Legislation, and Liberty which are generally regarded as Hayek’s most important contributions to the field of political theory.
According to Hayek, government is “necessary” to fulfill the following tasks: not merely for “law enforcement” and “defense against external enemies” but “in an advanced society government ought to use its power of raising funds by taxation to provide a number of services which for various reasons cannot be provided, or cannot be provided adequately, by the market.” (Because at all times an infinite number of goods and services exist that the market does not provide, Hayek hands government a blank check.)
Among these goods and services are
protection against violence, epidemics, or such natural forces as floods and avalanches, but also many of the amenities which make life in modern cities tolerable, most roads … the provision of standards of measure, and of many kinds of information ranging from land registers, maps and statistics to the certification of the quality of some goods or services offered in the market.