The purpose of this legislation, known as "Grant's Law," is to strengthen the process of detaining individuals who are unlawfully present in the United States and have been arrested for certain criminal offenses. The law aims to ensure that these individuals are held in custody while their legal status is determined, particularly if their offenses could lead to their removal from the country. Key provisions of the law include a requirement for the Secretary of Homeland Security to detain any unlawfully present individual who is arrested for specific crimes.