There are several reasons that Weatherbys have a double radius. the first was that Roy Weatherby was trying to get the most velocity, and from an engineering standpoint this should help for the reasons that sable tireur mentioned. and also he believed in lots of free bore to lower pressure and allow more powder.
The freebore worked but the double radius appears not to have made any difference in the efficiency. In fact it appears to have diminished the powder efficiency compared to some straight tapered designs. When you take a standard Weatherby chamber and change only the radius shoulders to straight angles (35 to 40o) and leave the freebore and lead angle the same, it appears to improve the efficiency by needing less powder for the same velocity.
The short magnums are a good example of the efficiency of the straight shoulder angles and a shorter powder column. I have many Weatherby chambered cartridges and like them but I have not seen any improvements in throat erosion over standard cartridge rifle barrels.
Not everything in theory works like it should, so until it is proven it is still just theory.
Just a note; If the radiuses were better, the 6 PPCs and other bench rest cartridges all the way up to the Chi Tacks would all have them.
Again, I have some very accurate Rifles using Weatherby cartridges but I also have some very accurate rifles in non Weatherby cartridges
so the radiuses are more of a trade mark of the Weatherby cartridges
than anything else in my opinion.
J E CUSTOM