Question

In the story "The Shady Plot",why did Helen appear in bits and pieces?

Answer 1

That is only to create a horror feeling on the readers and also for john

Answer 2

Well, this is a very tricky question. There is no mention of any reason as to why Helen, the ghost appeared in sections. Though, Hallock asked her why she materialized in sections, she gave no specific answer. 
The possible explanation to her appearing in sections could be this that it could be the protocol to materialize or vanish. It was Helen's general style of appearing and vanishing; whenever she appeared or vanished, she did it in sections. You can say it was her style.