Why did maniac bring his guest to Pipers birthday party?

Maniac brought his guest, Amanda Beale, to Piper's birthday party to show that he valued friendship and wanted to include her in a fun and social event. He believed that attending the party would help strengthen their bond and provide a sense of normalcy for both of them. Additionally, it was a way for him to demonstrate that he cared about the community and was willing to engage in local festivities, despite his outsider status.