Why oscilloscope is needed for circuit debugging

As handy as a DMM can be, unfortunately it cannot shows you circuit oscillation. DMM has a very slow response, for example, if the circuit shown below oscillate at frequency above 1kHz or more, DMM will just give the average reading, in this case -1V. Since you are expecting -1V, you would think that the circuit is all fine. But the oscillation will give faulty result to other stages. And at times like these, you will make the wrong conclusion and spend days on debugging.