On the use of hexane in the organic reduction by DIBAL-H
Unlike other reducing metal hydrides (e.g., $\ce{NaBH4}$ and $\ce{LiAlH4}$), diisobutylaluminum hydride (DIBAL-H) is a liquid at room temperature and dissolve in many hydrocarbons such as toluene and hexanes, which also have very low freezing points. For example, hydrocarbons toluene and hexanes both have freezing points around $\pu{-95 ^\circ C}$. Thus, they are good solvents for reaction involving reduction by DIBAL-H at low temperatures such as the example given in the question ($\pu{-70 ^\circ C}$). As shown in the equation, the low reaction temperatures help stop the reduction at aldehyde level. At room temperature with excess DIBAL-H, the reduction may continue further to give corresponding alcohols or even corresponding alkanes.
DIBAL-H reduces alkyl or aryl nitriles to their corresponding imines at low temperatures and resultant imines converted to corresponding aldehydes upon acid work-up. The same principle mentioned above apply here as well. Here, at high temperatures, imines reduced further to give corresponding amines.
Note: To see the mechanism of the reduction, see here.