Equilibrium and Completeness of Reaction
The basic stoichiometry is done to understand the basic concepts of moles and ratios in reactions. Later on, as the reactions become more complex, alot of them don't go to completion and so for a lot of them, you need equilibrium constants. This being said, not all reactions end partway, some of them do go to completion. The most basic example is disassociation of salt ions in solutions. From basic stoichiometry, you learn that the ratio is one to one (like NaCl -> Na+ + Cl-). So Moles of NaCl added = the moles of Na and Cl ions in water. So basic stoichiometry is sometimes (not always) applicable but is essential for the learning process.