Illinois was on the rise when they pulled in Harper. Henson had recruited Eddie Johnson and Mark Smith a couple of years earlier. Their sophomore year, Illinois started 15-0 and beat the number one team (MSU) with Magic Johnson before slumping after losing their point guard. The next year, they won 22 games. Then Henson added Harper, jc Craig Tucker, James Griffin to go along with Johnson, Smith, Derek Holcomb, Perry Range and they became one of the best teams in the league. After Harper's sophomore year, Henson got a top 5 class with Bruce Douglas, Efrem Winters, Doug Altenberger, Scott Meents and Henson really had Illinois back.
So, short answer, Illinois was already good when Harper committed, and he helped the program make another jump.