Why did the people of the annex need to light fires every other day?

Anne and the others had to light fires every other day in order to burn their refuse. It is too dangerous to cause any kind of suspicion by throwing it away.

Although it's undeniably hot, we have to light a fire every other day to burn our vegetable peelings and garbage. We can't throw anything into trash cans, because the warehouse employees might see it. One small act of carelessness and we're done for!