Usually it's so your opponents have to be more accurate by landing a higher percentage of headshots to get the kill, or in some cases, require more bullets. And in some rarer cases if your opponent is a bit out of range, the damage falloff they experience might leave you with a sliver of health rather than killing you. T10 resilience isn't really big enough of a difference for me to notice though, so I usually just go with 6 on my Warlock and 4 on my Hunter.