Why is it necessary to avoid even traces of moisture from a Grignard reagent?
Grignard reagents are highly reactive and react with any source of proton to give hydrocarbons. Even water, alcohols, amines are sufficiently acidic to convert them to corresponding hydrocarbons.

$\ce{RMgX + H2O -&gt; RH + Mg(OH)X}$

Where, $\ce{R}$ is a hydrocarbon.