
you don't have to package the nif file of that item into your mod, only the texture (.dds) files, decreasing the size of your mod file and making it easier to package.
The advantages of this system when retexturing a vanilla item are: Since Fallout 3, Bethesda introduced the use of Texture sets that allow to use only one nif file with different textures. In Oblivion, when you wanted to release a retexture of an item (armour, weapon, etc), you had to make as many copies of the nif files as retextures you wanted to create, and then link the different textures to each of the copies.
Creation Kit: Downloadable from inside the Launcher. In this guide I am going to add a new recolour for an armour, but if you want to retexture another item (like a weapon), it works exactly the same. They are a very useful tool for retexturing meshes (clothing, armours, weapons, bodies, etc) without having to duplicate the mesh files, and are a step beyond the Texture sets that were used in Fallout 3 and Skyrim. This mini guide will show you how to use Material Swaps for Fallout 4.