Is 21 percent really necessary?

Here is the drill. You save up all unopened bills for about 10 days. Then, one evening, you sit down at the desk, open each one, shake out the stuffers, check to see if last month's payment got there, circle the due date for this month's payment, tear off and discard the short mail-order form on the back of the return envelope. Then you bring your checkbook stubs up to date and start writing checks.

If you are like most, you are making monthly payments on longstanding balances - to stores, to banks, to credit card accounts, to credit unions or to finance companies. So, at one bill-paying session maybe, you notice that those outstanding balances haven't been decreasing quite as fast as they used to. And when you look closely (sometimes at the very fine print), you see that in many cases finance charges have been increasing. That means more and more of your monthly payment is going for interest (''finance charges''). And that means less goes for paying off what you purchased. And the payoff takes longer.

Major department stores now charge between 18 and 19.2 percent on the first $ 1,000 owed - 12 percent over that.

Some bank cards get 21 percent on the entire balance. That means on an account owing $1,500, a 21 percent monthly payment amounts to about $44. Out of that amount, 58 percent-plus ($25) goes for finance charges. That's sizable hot cash right out of your spending checkbook.

Everyone knows that it's rough to be buying so much on credit these days. Credit costs take too big a bite and make bills very difficult to pay off; and there's a great deal of personal concern connected with big credit bills in times of economic uncertainty. But the fact is, you and others are buying more and more on credit - regardless of what the finance charges are. Proof can be picked up from any late Federal Reserve Board report. Typical is the March issue: In it the January 1983 credit-buying total (revolving accounts other than autos) is shown as increasing by $2.93 billion - biggest monthly gain in two years.

And where do the finance charges come in?

At one time, state usury laws prevented the application of abnormal finance charges to credit accounts. But now, in many states, the limits have been lifted or certain kinds of credit activities have been made exempt.

And the credit control act of 1969 has long since expired - although some consumer groups are pressing for its reinstatement.

Today banks, retail stores, finance companies, and others say they need the up-to-21-percent charges to cover escalating management-and-money costs. Perhaps. But a reduction in finance charges would certainly help to slim down the family budget now for many consumers. And it might even accelerate all-out credit buying (which many economists say is a key factor in economic recovery). Look how car sales boomed when finance charges were reduced. The prime rate at this writing is down to 10.5 percent. Why isn't it logical to have a comparable reduction in those offshoot interest rates that would lighten consumer payments on monthly bills?

Isn't it time that another look is taken to see what is happening to the high cost of consumer-buying for credit?