While subsidies are one cause, another cause is demand response problems:
Imagine you have a greenhouse factory producing strawberries. The strawberries come out of the factory on a conveyor belt. Now, you can't just turn off this factory easily since it might take a week to get it up running again and you don't have any storage, so what happens when nobody wants your strawberries for a day?
You pay someone to take them.
And yes, it happens to physical products too, e.g. household electronics - you need to pay someone to scrap them.
A better example would be plastic production. In an emergency stop, factories need to change several pipes because the plastic solidified inside. Stopping production properly takes days, and even then, resuming it also takes days.
Imagine you have a greenhouse factory producing strawberries. The strawberries come out of the factory on a conveyor belt. Now, you can't just turn off this factory easily since it might take a week to get it up running again and you don't have any storage, so what happens when nobody wants your strawberries for a day?
You pay someone to take them.
And yes, it happens to physical products too, e.g. household electronics - you need to pay someone to scrap them.