Contrasts can be used only when there are clear expectations BEFORE starting an experiment, and these are reflected in the experimental design. Contrasts are planned comparisons.
Patients are given either drug A, drug B, or a placebo. The three treatments are not symmetrical. The placebo is meant to provide a baseline against which the other drugs can be compared.
Multiple comparisons should be used when there are no justified expectations. Those are aposteriori, pair-wise tests of significance.
We compare gas mileage for eight brands of SUVs. We have no prior knowledge to expect any brand to perform differently from the rest. Pair-wise comparisons should be performed here, but only if an ANOVA test on all eight brands reached statistical significance first.
It is NOT appropriate to use a contrast test when suggested comparisons appear only after the data is collected.