Lakshmana cuts off Surpanaka's nose in the Hindu epic Ramayana because she had approached him and his brother, Rama, with romantic intentions and had threatened Sita, Rama's wife. Surpanaka, who was a demoness and the sister of the demon king Ravana, was attracted to Rama and desired him as her husband. When Lakshmana rejected her advances and warned her to stay away from Rama and Sita, she became enraged and attacked Sita. In response to this threat against Sita, Lakshmana defended her by cutting off Surpanaka's nose.
The direct consequence of Lakshmana's action is the disfigurement of Surpanaka, who is left with a mutilated nose. This act also serves as a warning to Surpanaka and other enemies of Rama and Sita, establishing the boundaries of their protection.
Indirectly, this incident leads to a chain of events that eventually culminate in the epic battle between Rama and Ravana. Surpanaka, seeking revenge for her disfigurement, approaches Ravana and instigates him to abduct Sita, leading to the conflict between Rama and Ravana. Thus, Lakshmana's action plays a role in setting the stage for the larger narrative of the Ramayana.