Why use Km in catalytic efficiency?
The most basic kinetic scheme for enzymes is represented as
$$\ce{E + S &lt;=&gt;[K_m] ES -&gt;[k_{cat}] E + P}$$
As should be clear, the $k_{cat}$ is the rate constant for the reaction that occurs after substrate is bound to the enzyme. The resulting rate (kcat[E]tot) is only achieved when every molecule of enzyme essentially always is in the act of converting substrate to product. That is, every time a product molecule is released, a substrate molecule immediately binds. Even if the substrate molecule dissociates before reacting, another immediately takes its place. We describe this situation as the enzyme being &quot;saturated&quot; with substrate.
The Km is a measure of how tightly the substrate binds to the enzyme, approximately equal to the equilibrium constant for the dissociation of the substrate from the enzyme. If the substrate is at a low concentration relative to this Km value, then many of the enzyme molecules will not have substrate molecules bound to them and will be unproductive as a result. The overall rate will be substantially below the maximum  kcat[E]tot.
All of this is captured in the basic Michaelis-Menten kinetics equation:
$$Rate = \frac{k_{cat}[E]_{tot}[S]}{K_m + [S]}$$
You can see that the impact of $K_m$ on the rate increases substantially as [S] decreases relative to $K_m$.
Since most substrates exist physiologically at concentrations below what is required for maximum rate, it is the combination of $k_{cat}$ and $K_m$ relative to [S] that determines the in vivo rate of reaction (along with the amount of enzyme of course).