Corey -House synthesis vs Wurtz reaction
As $\ce{R-Li}$ is very reactive ( even more than $\ce{RMgX}$ ). To decrease the reactivity of $\ce{R-Li}$ , it is reacted with $\ce{CuX}$ to form Gilman Reagent which is less reactive than $\ce{R-Li}$ to produce alkanes. 

If  $\ce{R-Li}$ reacts directly with $\ce{R'-X}$, there will no formation of alkanes, but interchange reaction occurs. 

$$\ce{RLi + R'X --&gt; RX + R'Li}$$