It can be a factor when an independent school is reviewing your application. The SSAT provides a way for schools to compare students from different academic backgrounds.
Some folks claim that the SSAT is not a big factor when getting into a high school. However, the fact remains that the top schools have high SSAT averages. Your other numbers (GPA for example) not only test how smart you are, but how well you work. The SSAT can be a good indicator of your core skills.
Of the three major factors in school admission (your GPA, extracurriculars, and test scores), the SSAT is the only one that you still have total control over and takes just a few hours to complete. That is why it is so important to focus serious time and effort on preparation for this test.
What is the SSAT? What is a good score?