It is one thing to write down specific receivables, but what if you also have a considerable amount of smaller receivables (both the balance in total and the quantity of balances it comprises of)? You’re expected to assess each individual receivable balance when considering their collectability, but it’s not feasible that you go over each say 100 balances. 

So as such, it’s suggested that for the rest of the balances that you decided not to assess individually, you assess the collectability and the need for an allowance based on previous general experience and losses incurred from receivables not collected and written to expenses. If you have no previous experience, it’s more difficult obviously, but be critical in considering not recognizing an allowance. Some of your receivables are more likely to be “bad” and you may face issues when trying to collect them.