Or you actually know exactly what you're doing, but the complexity and cost of the system under development just requires that it be tested virtually before it gets to a certain point in production. Maybe you don't even have the parts, and all you have is a great idea in your head that you don't want to lose. You don't know exactly what parts will work, so you want to do some swapping and testing. And sometimes you are just fiddling around, playing with a new design and nothing is set in stone yet.
Each component has it's own parameters, even if you get two of the same part. Physical circuits built with real components can be fickle and difficult at best. If you're not yet familiar with Multisim, or any other circuit simulation software for that matter, let me give you a brief introduction.
