If you're an intelligent guy use Annuity and Perpetuity formula directly. It automatically auto calculates a left out value when other values are filled.

If you're an intelligent investor or agent, you can directly make use of Annuity /Perpetuity mathematical equations to fill any left out value when other values are filled in.
The left out values may be filled using iterative calculation.

It helps to accurately measure present value, future value of any loan while the calculators available online is for the masses which aren't intelligent enough nor do they give enough flexibility.

For example in case of Annuity you can take suggestions from your customer say he can only tell that he is ready to pay X amount every month while loan interest rate is 1% per month and present loan value is Y amount, then how many months will it take to close this loan? He may like to ask in another way say he now only wants how much interest will given other inputs.