Why is thionyl chloride preferred for preparing alkyl chlorides from alcohols?
Thionyl chloride is preferred for preparing alkyl chlorides from alcohols because the by-products formed in the reaction are $\ce{SO2}$ and $\ce{HCl}$ which are in gaseous form and escape into the atmosphere leaving behind pure alkyl chlorides.

$$\ce{CH3CH2-OH  + SOCl2   →  CH3CH2-Cl  +  SO2 ↑ + HCl ↑}$$