Why you might choose to use the method of standard additions, rather than create a calibration curve, as part of your method to analyze a sample?

This is because the Standard Addition is more likely to give a more definite measurement of the target concentration of analyte in the sample as compared to employing a calibration curve.

In a situation where sample matrix also gives to the analytical signal and establishing a situation called the matrix effect, Standard Additions is the best option to employ rather than calibration curve.