Suppose you have a chip such as a 555 timer, which creates square-wave oscillations. Ideally the output should be a nice, clean trace such as the one in Figure 2, but in the real world the output may be more like Figure 3. The transient voltage spikes can cause problems elsewhere in a circuit, especially if they enter the power bus. If you add a capacitor between the positive supply pin of the chip and negative ground, this may help to suppress transients. But which value of capacitor will do the job most effectively? An oscilloscope can answer that question.