Sound urban planning is extremely important. From the perspective of human history, urban planning is permanent. It is an extremely rigid medium. Nothing short of catastrophic destruction can hope to alter city plans once implemented.
The urban planning we have today, if we have anything we can even call urban planning, is often abysmal. Developers can basically build with no thought or care given to the long-term consequences of their actions (which, frankly, is what city planners should be concerned with, but alas).
One example: strip farmland, near or in cities, rezoned for development. Strip farmland is divided into narrow strips with unpaved country roads offering farm equipment access to the farmer's crop. This makes sense as narrow strips reduce the distance a farmer must travel down the country road to get to his plot. It also takes fewer turns of the farm equipment to harvest said crop.
The problem, however, is that developers then build for that narrow plot in isolation. That means a dead end street is paved off the (probably by now paved) country road. The process repeats as more farmers sell off their individual plots. The result is a comb configuration.
Why is this a problem? Well, because now you have all these little dead end streets that feed into one, long, skinny country road. Because of the lack of proper urban planning, residents are forced to drive to stores, work, schools, etc. because none of these were included by developers in these developments, leading to traffic congestion. And if they were included, it still sucks to navigate the terrain because the country road is the only traffic artery, and it risks turning into a so-called city "stroad" that is hostile to pedestrians.
You could mitigate this to some degree with some very modest regulation of land use. For example, the selective use of eminent domain to prevent building N meters from the narrow ends of the plot to allow for future street construction (to complete the comb into a grid) or non-residential buildings like stores, schools, and businesses.
Of course, developers can screw up large plots of land just as well, so at least the strip farm limits his reach. The point is that, whatever, the case, the city or town must not allow developers to _dictate_ urban planning.