Thomas Aquinas asked: what is the point of laws? We know bad people will ignore them, and good people won't want to do those things anyway, so what good do laws do? His answer was that laws help in the marginal cases -- people who might do bad things, but who are deterred by laws.
It is important to keep in mind that laws do not work perfectly, as Aquinas noted. While some will change their behaviour to obey the law, others will never have wanted to do what the law forbids in the first place, and still others will continue their behaviour even if it violates a new law. Another group of people will attempt to continue their behaviour within the bounds of the law by looking for loopholes, exceptions, and work-arounds. That will lead to calls for more laws to close the loopholes.
More laws come at a cost. There is an administrative cost, of course, but there is an equally important civic cost: we expect people to obey the law, and to obey it, they have to know what it is -- ignorance of the law is no excuse. By multiplying the number of laws, we impose a burden on citizens who are obligated to know them.
The civic cost also includes a loss of freedom. For many laws, such as murder and burglary, this is not a problem; no one should have the freedom to kill or steal. But other laws prevent things that would be fine if only a few people did them. No one objects in principle to people rescuing baby seals or baby deer, but there is a danger of too many people interfering in wildlife. We resort to laws that allow only licensed professionals to engage in such activities, but licensing is only a crude tool for limiting which people are allowed to interact freely with animals. How much better if people observed the limits on their own; if only a few people used bad judgment, it would not cause a crisis, and we would need no law to prohibit well-intentioned people from doing good deeds.
Laws have to be made to fit the character of the people they would govern, as Montesqueieu argued. A government can impose unpopular laws, such as Prohibition, but only with an enormous enforcement effort. Alternately, we can overlook violations of unpopular laws. But that option is worse, because it tells people the laws aren't
important. If you're not going to enforce a law, don't create it. I have always thought there is something fundamentally wrong with our speed limit laws for this reason. Clearly, few local or state governments think the speed limits they set are really important, because they don't enforce them until someone is going at least 10 mph above the limit. They should enforce the limit posted, or raise it until they think it is worth being enforced.
Have you ever wondered why the IRS expects you to fill out your own tax return? It opens the door for people to cheat in many ways, and the IRS can only audit a small portion of returns; and, naturally, they focus on the ones that exhibit behaviour characteristic of other tax cheaters (such as claiming large charitable deductions). But what an administrative nightmare it would be if the IRS had to calculate everyone's taxes for them. It would basically be an unworkable system: the government would have to hire tens of thousands of more employees to interrogate you about your sources of income and your deductions, and then fill out the forms. The IRS trusts you, more or less, to do the job yourself because it has no choice. Moreover, much as most people hate filing taxes, I have no doubt that they would hate the alternative of direct IRS participation even more.
Laws are like filing taxes. The government relies on people to obey laws on their own; police forces and enforcement agencies are like auditors that go after the people -- hopefully a small number -- who don't police themselves. If people don't believe a law is just, the government is required to take over enforcement in a centralized fashion instead of "outsourcing" the job to individuals. This is a very costly way of governing, and imposes a huge burden on the citizenry.