Higher IV Pokemon do better in any circumstance compared to lower IV Pokemon. Besides the fact that CP is higher at good IVs, also the Individual Values themselves are better. That's why higher IV Pokemon are better. A 100% IV Pokemon has all its stats maxed out (Atk, Def, Sta, Sp Atk, Sp Def, etc.). In comparison to a 80% IV Pokemon who has its stats at 80% out of 100%.