"In order to test plumbing pipes for iron material. Magnets are often used to check plumbing pipes for ferrous materials, such as iron. By running a magnet along pipes, inspectors can determine if they are made of materials that are susceptible to rust or corrosion. This helps in assessing the condition of the plumbing system and identifying potential issues. The other options mentioned are not common reasons for home inspectors to carry magnets during inspections."