I use simulation for a couple of reasons.

Last year I was given and existing design that didn't work.  i found a few
problems like op amps going into saturation and taking way too long to recover.
In order to develop a fix I used simulation to determine if the voltage limiter
would effect the signal enough to be a problem and if the tolerances on
resistors would vary the gain to be a problem. This was very useful and the
circuit worked properly when built.

Earlier this year we had a customer tell us that our ADCs were noisy because the
signals from their equipment with the wires running across the floor near a
noisy AC dynometer to our ADCs looked noisier than their existing system with
the wires in conduit. I found that our ADCs had no antialiasing filters and used
simulation to add filtering and verify that it would not effect the signals of
interest. The filters when built performed as the simulations predicted, but the
customer still complained that our signals were noisy. Then I simulated their
ADCs which were supposed to have a 40KHz passband, and found that they had a
26Khz passband that started to roll off before 1KHz! When they finally tested
their system (which had been in place for 5 years) they found that the
simulation matched the system and realized why their data was so dead.

I have also used quite a bit more often simulation of PLDs and FPGAs toe verify
their timing would actually work, and I wish the people who designed the boards
I am supporting had done much more so I wouldn't have so many problems to fix.

Simulation is a tool that can make your life easier, but it is only as good as
the input given it. If the models are wrong or it is applied without knowing how
the circuit should operate it will mislead you. I had this happen when I tried
to use an Opto to isolate an analog signal. The first attempt looked like it
would work, but my fellow engineer made me look closer and I found that it was
running in such a narrow band any noisy would overcome the signal. I went back
to the drawing board and came up with a better circuit.  So simulate to optimize
the circuit, eliminate possible designs that won't work, and determine what
variations in the parts will do to the circuit, then build the thing to verify
in the real world.

Rob



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