Is it effective for an author to imitate someone else's writing style? Why or why not?

Possibly. If said writer appeals to you, as well as a large number of readers, then their work and style has substance.

Imitating their writing style should appeal to a similar group of readers.

The problem here is that it will be difficult to mimic the style EXACTLY. Nor do I think you should copy it exactly. Let your writing style be YOUR writing style. Chances are it will have bits and pieces of all the work you have ever absorbed.

You never know. Your style might appeal to a larger audience than the original writer you were trying to copy.