Sometimes I read things on the internet written by libertarians. I used to read reddit a lot and that has always been a bit of a libertarian stronghold. Occasionally I read things on reason.com. I think I'm pretty squarely on the left side of most political systems but libertarianism is one part of conservatism that I do have some sympathy for. I like the idea of legalizing all drugs, for example, not getting into wars without a really good reason, and generally not imposing unnecessary rules on people or companies.
Those who describe themselves as libertarians often seem to take this principle to extremes, though. One sentiment in particular that I've noticed being expressed repeatedly is that all taxation is theft. I disagree with this - theft is illegal while taxes are legal. Also, one has a say in taxes (via voting and other forms of participation in the political process) but no say in getting robbed. Finally, one doesn't get anything back from getting robbed but taxes pay for useful government services and infrastructure. To me the accusation seems like an instance of the worst argument in the world.
Another thing that libertarians say is that government has a monopoly on (legal) aggression and violence. That sounds like a bad thing (because monopolies are bad, and violence and aggression are bad). But in this case two bads make a good - you don't want multiple violent organizations competing, as that would not tend to minimize the amount of violence. Since we can't eliminate legalized violence altogether (since otherwise we would have no way to arrest an uncooperative murder suspect) it's best that a single (accountable) organization has that monopoly.
There are other things that government has a natural monopoly over - things that benefit society as a whole but won't get done by the markets because there isn't any profit in being the one to do them - things like making sure the poor have enough to eat and access to life-saving and preventative medical care. Another case is where having multiple competing organizations would cause practical difficulties: I don't want my house to be connected to six different electrical networks, six different water supplies, six different sewers, six different telephone networks, have driveways connecting to six different road networks, have six different garbage collectors coming by and I don't want my town to be served by six incompatible rail networks. The basic provision of utilities like these is best done by monopoly - even if building, servicing and billing can have competing providers (here in the UK I can choose from many different electricity companies to bill me each month, but they all have the same number to call if there is a power cut). Similarly, the military is probably best done centrally, otherwise different militaries representing different interests of the same country might end up fighting each other!
It makes sense to have one single organization take care of all the things that need to be done that can't or won't be taken care of by markets for one reason or another, and that organization is what we call government.
There's a similar concept in software engineering called a singleton object - a single chunk of memory where you put all the things that there is only one of in the program. It's bad engineering practice to stuff in the singleton that doesn't really need to be there, because it leads to inflexible programs that allow you to have only one at a time of something that you might want to have multiple instances of. Similarly, it's bad practice for the government to do stuff that the market can do better - I wouldn't want government to get into the business of designing and making laptops, for example, since the market does a great job at that.