Role of acetone in the Finkelstein reaction
Finkelstein reaction is a halogen exchange reaction between haloalkane and a salt of a different halogenide. $\ce{R-CH2-Cl + KI -&gt; R-CH2-I + KCl}$. The driving force in this reaction is precipitation of $\ce{KCl}$. It is important to find a solvent that would dissolve $\ce{KI}$ better than $\ce{KCl}$. Then solution will always have $[\ce{I-}] &gt; [\ce{Cl-}]$ (more iodide anions than chloride anions). As a result this equilibrium will be driven to $\ce{R-CH2-I + KCl}$. Acetone is exactly this type of a reagent.

For this case Le Chatelier's principle means that reaction will be driven to the direction in which one of the reagents gets outside of the solution. Since $\ce{KCl}$ keeps precipitating the reaction will be driven to the desired product.

This reaction is important because there are many ways to make $\ce{RCH2Cl}$, but making $\ce{R-CH2-I}$ is difficult. Finkelstein reaction allows to convert available $\ce{R-CH2-Cl}$ to $\ce{R-CH2-I}$.