Yes, there are certain processes that can and do use that much ram.: anything that involves analyzing an EXTREME amount of data. Take the LHC...in 10 GB of data (relatively small because all the detectors spit out terabytes of data per second), there's 60,000 events containing hundreds of particles with each particle having hundreds of parameters (momentum, energy, etc). Whenever I run some analysis on the data it needs to load up all 60,000 events...and I promptly max out use the 3 GB of ram available to me. In case anyone is wondering, even a relatively simple analysis requires about an hour-hour.half to run over that many events. It's not so much cpu intensive as it is memory and hard drive intensive.
In terms of engineering I imagine simulating a building collapsing or something would require a lot of RAM as well.