What, specifically, does an MIU grant a Techpriest that his Mechanicus Implants trait does not? Is it simply a superior method of interacting with machine spirits over his Electoos?

If you're talking about the Talent "Electro Graft Use", then there is certanly alot of overlap between it's functionality and a MIU.

This is quite common, there are many talents for Tech-priests which have similar functionality to implants that other classes may purchase if they have the funds and can find the implants. The tech priests just have to invest the XP.

Though there are some advantages for the Tech priests too. An MIU can give bonuses when interfacing with weapons or vehicles, which the Electro Graft do not.