Lab Examination of Clothing

Why is it important that law enforcement catalog everything on a subject when they are arrested?

After Timothy McVeigh was taken into custody, the FBI recorded the results of the FBI Laboratory High Explosives Examination on his belongings. They tested for PETN, ammonium nitrate and fuel oil that were used to create the bomb. Ammonium nitrate was detected in McVeigh’s pant pockets and earplugs at the time of his arrest, linking him to the bomb.