What happens when you spend a few hours breadboarding a new circuit and you accidentally miswire something, creating a short, and then you connect this circuit to a unregulated supply with no current limit?
What happens when you want to charge a lithium ion battery, which needs an inital charge at some given constant current and then needs a final slow charge at a given constant voltage?
And of course you can't control voltage and amperage at the same time. That's physics. Think ohms law - for a given resistance and voltage, current is fixed. There's no way you can force a circuit to pass more or less current than ohms law allows, and the same with voltage at a given current.