What were they planning with attacking Sidney at her house?

the anniversary was the big event so..
it's just a stupid scene, that could've just been the phone call instead of ghostface scares

Billy needed to give himself an alibi to throw off suspicion on him for any future murders and gain Sidney's trust so she would be more vulnerable on the night of the anniversary.

They did it just to mess with her.

I believe its been stated by either Craven or Williamson that the purpose of the attack was to just to toy with & scare her.