What is the use of uncollectible accounting method?

Based on experience, a company will know that a certain percentage of their outstanding accounts receivable will be uncollectible, They apply this experience via a formula to approximate the dollar amount of uncollectible ,and set up an allowance for doubtful accounts (a contra-asset account) and the debit goes to bad debt expense .The formula might be as simple as 1% of total A/R or various percentages applied to an A/R aging (50% of over 90 days old +10% of over 60 day old etc.).At the end of each period, the allowance is adjusted and re calculated accordingly, up or down with the offset to bad debt expense.