GCSEs or IGCSEsIGCSEs are taken by many independent schools in preference to GCSEs because they are frankly, harder, and a better preparation for A - Level or IB.
Many people think that the standard of GCSEs has fallen since it was introduced, partly because of the introduction of school league tables and the push by many schools yo have students get 5 GCSE grades C - A*. Exam boards has been in competition with each other to have schools sit their exams in preference to other boards, and one of the ways in which they have competed is to lower the standard needed to pass their exams. Exams being easier to pass, but still counting as a GCSE would therefore be preferred.
Competition between exam boards for IGCSEs is less intense because it is not as common as GCSE s and standard have been maintained. In maths IGCSE for example, there are many topic areas from A - Level Maths eg integration and differentiation.