To allow the loaded cartridge to chamber in different guns. Are you asking about them shortening the front driving band, or their reduction in diameter. They did both. The reduction in front (driving) band diameter was to allow easier chambering in revolvers with tighter cylinder throats. Early Colt DA revolvers often had .355 to .356 throats, and bullets with a .358 driving band were difficult to chamber. And even worse once the chamber fouled a bit. The shortened and reduced driving band made chambering easier under these circumstances.