What are the specific applications of acid-base equilibrium?

There are many aspects of acid/base chemistry that can only be explained by having an understanding of equilibrium. Here are a few that come to mind.

1. We can't explain or predict the pH weak acids (or weak bases) without reference to acid-base equilibrium.
2. Acid-base indicators undergo a colour change over characteristic pH ranges. This is because they are often weak acids (or bases) that have different colours in their un-ionized (molecular) form versus their ionized form. Acid-base equilibrium and the concepts of Ka (or Kb) allow us to explain (and predict) the colour change pH ranges of the different indicators.
3. Titrating strong acids with a strong base will yield a neutral solution at the equivalence point, but weak acids titrated with a strong base will have a basic equivalence point, while weak bases titrated with a strong acid will have an acidic equivalence point. The concepts of acid-base equilibrium allow us explain these observations and predict the equivalence point pH of different titration scenarios.
4. The behaviour of buffers can only be explained and understood with the concepts of acid-base equilibrium. Acid-base equilibrium concepts also allow us to create buffers of a specific pH as well a predict the new pH of a buffer upon the addition of small amounts of acid or base.
5. Some salts, when dissolved in water, will change the pH of the solution from neutral to slightly acidic or slightly basic. This can be explained and understood by considering conjugate acids and bases and the concepts of acid-base equilibrium. With these concepts, we can predict if a given salt will have acidic, basic or neutral properties.