In your example, "up" has an emphatic function, and suggests "completeness" in the action.
1. I broke his toy plane.
you might say this if you had e.g. broken a wing.
2. I broke up his toy plane.
you might say this if you had completely dismantled the plane, into separate pieces.
This is not a general rule, however; different prepositions may have different functions in different contexts.
Student or Learner