See how extra credit changes your grade.
Extra credit expressed as percentage points on the final grade behaves differently from bonus points inside a points total. Two things blunt it: you rarely complete all of what is offered, and most courses cap the grade at 100, so points earned above the cap are simply lost. Modelling completion share and the cap gives the gain you will actually see rather than the gain advertised, and the wasted-points line tells a student already near the cap that the effort is better spent elsewhere.
Points added
Points added = extra credit offered x share you complete
Capped result
Final grade = min(grade cap, current grade + points added); the excess is lost
Because of the cap. A student on 98 offered five points can only gain two, so the remaining three are wasted effort — which is exactly what the wasted-points line quantifies.
Usually, if you are below your target. Even half of a five-point offer is two and a half points, which is often the difference between grade bands at the boundary.