Hi L,
Understanding sufficient and necessary assumptions is primarily important when that wording appears in the question stem (and then produces a Justify or Assumption question). At that point, the short answer is that while the two types of questions often have identical answers, they don't have to have identical answers. Sufficient assumptions can contain "extra" unneeded info that would make them incorrect in necessary assumption questions. So, if you don't know the question type you are facing, you might eliminate the correct answer and choose an incorrect answer
Please let me know if that helps, or if you would like to discuss it more. Thanks!