You select the ISO _or_ aperture on the meter and then it will tell you the
shutter speed for proper exposure given the ISO/aperture input. You can then
fiddle with either aperture or ISO values using the dials and it will tell you
the corresponding change in shutter speed to get the same exposure. The
better flash meters like my Sekonic L-358 will also allow you to fire the flash
and take differential samples from various angles to come up with an average
exposure...it will also tell you what percentage of the exposure is being controlled
by the flash setting too
Wonderful tool....but not a necessity. It's nice to point the thing and have it
tell you what to shoot if you're not sure of the settings or don't have time
to 'chimp'. Mine was handy for a night shot I wanted in Vegas...I had no idea
I needed 20 seconds of exposure for the given light and I was in a hurry..so
it came in handy.
You can go your whole life without having or needing one but they're a neat
tool to have in the bag.