Our particular project is about analog simulation, though DARPA is also funding surrogate modeling efforts in the digital domain. On analog in particular, one significant impetus is speeding up mixed signal simulations, since digital simulators (in terms of seconds of execution simulated per seconds of simulation time) are much faster than analog simulators, so if you want to do a mixed signal simulation, you're generally running it at the speed of analog simulators. For end-to-end simulation, it would be much nicer if you could bring up the speed of the analog simulators to be comparable to the digital simulators. Of course there is a question of how much accuracy you're paying (and we're still finding out what the tradeoff is), but in the kinds of simulations where this comes in, your actually often ok with losing some accuracy, since you can always get some noise coupled into the analog portion, so your control electronics should be robust to slightly weird analog signals anyway.