Why is a reaction more favorable if one of the products is not soluble/sparingly soluble?
If the reaction product is removed from the phase (liquid solution for example) in which the equilibrium reaction is occurring, significant amount of product can be obtained even if the equilibrium constant for the forward reaction is relatively small ($K&lt;1$).

Aside from precipitation, product could also be removed by vaporization, with subsequent condensation (distillation) to collect the product.  Anything that reduces the product concentration in the phase where the equillibrium occurs favors the forward reaction.