Because there may be kits behind it. NYSDEC notes female raccoons look for den sites in late winter and give birth in April and May, and the young open their eyes about three weeks later and nurse for roughly six weeks before leaving the den. Seal the mother out during that window and the kits die in the insulation, and she tears at the roof, the soffit and the vent trying to get back to them. NYSDEC's test for whether the den is empty is simple: sprinkle twigs, grass or flour at the opening and watch for tracks, and listen. Young raccoons, in NYSDEC's words, often announce their presence with mewing, twittering or crying sounds.
The full walkthrough, including what New York law allows and the spring timing rule, is in our guide How to get rid of raccoons in the attic on Long Island.