Field Oriented Control Feed Forward Term
Consider an R-L load, the transconductance transfer function is as you wrote:

\$ \frac{I_{out}}{V_{in}}= \frac{1}{Ls + R} = \frac{ \frac{1}{R}}{\frac{L}{R}}s +1 \equiv \frac{K_{dc}}{\tau s + 1}\$

This can be viewed as your plant model P(s) and you can then create a controller C(s) to control the current in this RL load

\$ \frac{Y(s)}{X(s)} = \frac{G(s)}{1 + G(s)H(s)} = \frac{C(s)P(s)}{1 + C(s)P(s)H(s)}\$

Your controller could be a PI controller to minimise any errors in your load and equally control the bandwidth of the overall response. A voltage is applied to the load to realise the needed current.
At DC the voltage needed is that to overcome the resistive component, at AC it is the voltage needed to overcome the resistive and reactive component and the controllers response aims to control this.

The plants terminal voltage is thus:  \$ i(t)R + j\omega Li(t) \$

But what about synchronous machines? (wound rotor, BLAC, BLDC? ). Current is needed to produce torque to either accelerate an inertia or oppose some load. With increase velocity the backEMF increases. This is a disturbance into the current loop. The terminal voltage is now:

\$ i(t)R + j\omega_e Li(t) + K_e \omega_m\$

So the current controller now needs to demand more voltage for the same torque. Under a constant speed situation there is no problem and is obvious you need more terminal voltage while running at a given speed to overcome the backEMF. But what about under dynamic responses? bandwidth testing? higher loop disturbance rejection? The current loop's is no longer outputting a response needed to produce torque but to mitigate a disturbance and this decreases the currentloops response. 

This disturbance however is well understood and thus can be mitigated via a feedforward term to "inject" a voltage demand that would be known to be needed to overcome the backEMF at the present velocity, this leave the current loop to respond purely to the torque demand, be it due to load changes or demand changes 

You have already included a diagram at to where the feed-forward term would be injected (I usually do not inject on the d-term unless I am employing some form of field weakening) and hopefully I have provided at least an initial insight as to how you may benefit from it when you have outer control loops and system dynamic's to control.