Most models from fab or marketplace are not ready. Esp, materials, they made to look good on pictures, not so much to work good in Unreal. Yes textures are fine, just materials not always (mostly auto generated on import).
You also want single master material for same type of surfaces (like same material for all hard surface meshes like tanks, guns etc)
For eg if you have multiple guns from different packages and want to mix them (ie. kitbash them all together). You want/need single master material and then set correct textures in instanced materials.
Another example:
There is really nice set of packages from single author (i love it) however each package has slightly different master material, when applying same parameters value (like metallic or emmisive) those materials look different. There are some parts like shoes in separate packs, have same textures, but different version of material, and those shoes look different (again because of different material).