It depends on the specific scenario. For parking, for example, not charging for it when it's in high demand is generally a bad idea, because you get "overconsumption": people who barely even need it end up using it anyway (hey, it's free!) while people who really need it have a hard time finding any available. So you'll have, say, people who are just storing their occasionally-used car for weeks between uses on the street, while people who are just parking to unload something right now can't get their stuff done.

Then you also end up with people spending a lot of time circling around downtown looking for elusive free parking, which is bad for both traffic and the environment. In contrast, charging a "market rate" that usually leaves 10-15% of parking spots open means that scenario is now transparent and fast: you know you can usually quickly find parking, you know how much it's gonna cost, you can make the calculation ahead of time and execute fast.