It works like this:
Without WK, you can only attach a single mod to any weapon or armour. However, if the second mod would result in a basic assembly, you are allow to assembly it, which means you can construct all the basic assemblies without any levels in WK.
With WK1, you can attach up to three mods to weapons and two to armour, which allows you to assemble advanced assemblies, and with WK2 you can attach up to five mods to weapons and three mods to armour, which allows you to assemble the master assemblies.
WK2 has an additional benefit, that any assemblies assembled after getting it can be modded once with any mod.
It took me quite a while to figure that out too.