Acetal protecting group methodology is commonly employed when a reagent that is reactive to aldehydes/ketones needs to be used in the presence of an aldehyde or ketone functional group, but the desired reaction is at some other place in the molecule.
Acetal protecting groups are especially useful because they are easily installed with mild acid, inert to nucleophilic reagents, and easily removed by treatment with mild aqueous acid. The questions below relate to the following sequence of reactions depicted below.
The main reason that one cannot treat compound A with $LiAlH_4$ to form Compound D directly is
Compound B is structure
Compound C is structure
Structure Choices for Compounds B and C: