what is significance of piggy's plea to join the 'expedition.'

If this refers to the initial plea to join the group of boys, in the first chapter, it sets up themes for the entire novel: the idea that Piggy is a bit of an outsider, has to ask permission, is the (rejected) voice of reason, provides glasses that can be used to produce fire, etc.

If this refers to his request to join the visit to the other boys in Chapter 9, the meaning is related, but more focused; it shows him supporting Ralph's residual authority and sets up the final ruptures in civilization by pushing an accidental confrontation.