A senior session is an investment in memories. Graduating from high school is an important milestone in your life, which is why it's so important to document this moment. Photos can be printed, hung, shared with family, posted to social media, and featured in the year book of the graduating senior.
Graduation pictures can happen at every stage of academia. High school isn't the only time senior photos can be taken. Any milestone can be captured and look back upon for years to come.