If you're curious, the "dummy key" is a bump key. It's a relatively crude tool and is relatively hard on the lock, compared to traditional picking. There's also a certain level of professional pride, as you observed.
Hitting it with a mallet imparts force on the upper half of the pins, causing them all to briefly jump above the shear line, and at that moment the lock can turn freely without binding a pin.
A vibrating pick gun does essentially the same thing in a somewhat more controlled way.
After spending a number of minutes fiddling with some impressive-looking lock picks and exercising what must be years of learned skill, he couldn't unlock the door. Sheepishly, he put away the specific tools and pulled out a dummy key + a rubber mallet.
The locksmith put the dummy key into the lock tumbler, whacked it once on the head with the rubber mallet, and the door unlocked. From the time he produced the tools to the time I was inside the house was well under sixty seconds.
I thought, Why didn't he just do that in the first place? And concluded much the same as you've said -- $70 for thirty seconds of work might seem steep and some people need the song and dance. Personally, I was grateful to have on-demand service like that available so quickly :)