Most factory recoil lugs on say a Rem 700 are not heat treated, in fact they are quite soft if you have ever machined one. In a heavy kicking rifle like the 338 Edge, this can allow the factory lug to flex under recoil. Although this flexing may be VERY slight, it still happens and will effect down range consistancy.
Also, factory lugs are rearly even in thickness. You can spend all the time and money you want truing receivers and fitting barrels properly you want and if you sandwich a recoil lug between the receiver and barrel shoulder that is uneven, you will induce a stress into the entire rifle system.
Most after market lugs are surface ground and most are held to within 0.0002" in thickness uniformity. Also, most are heat treated and thicker then factory lugs for added strength and no flexing at all. Those that are not heat treated are generally very thick which accomplishes the same thing of no flexing just in a different way.
For a custom rifle
, if you do not use a quality aftermarket recoil lug, your really shooting yourself in the foot. Remember the saying the weakest link in the chain. THat is good to remember when building an extreme accuracy long range rifle.
For the money, very wise upgrade to have on a custom rifle