Hello Amruta,
I'll ask very carefully if you have a specific reason for using persistent classes?
I ask, because you need to understand them well to make proper use of them. If you did, you probably wouldn't need to ask this question. If you aren't fully up to speed on persistent classes it is definitely easier to work without them.
The BI_PERSISTENT interface is the persistence layer used by Workflow. In FIND_BY_LPOR, your job as developer is to instantiate a class that implements the BI_PERSISTENT interface and return it. In your instantiation you're free to code what you like to provide the class with data: SELECT, RFC, web service, persistent class, cookie, whatever, or even nothing at all. Workflow doesn't care, it just wants an instance.
There are two ways to use persistent classes: CL_BOOK_PPF is an example of a workflow enabled persistent class. The other way is to use a persistent class as a data source. This is how the blog shows it. A couple of variations are possible, but in principle you are performing your data access with a persistent class, and for simplicity the persistent object is saved in an attribute.
Regards,
Mike