you use simulation tools to get an idea of what will happen when you build the circuit for real. you don't simulate for fun. if you want to build a circuit and want to see if it will work or not, or if you want to see if it will work the way you want it to be, you use a simulation tool to verify. its easier to simulate first an hdl code before you send it to the manifacturer and have all the chips manifactured and see that they don't work. so you simulate first to fix the errors and get a working design manifactured.
the power of the tool depends on the level of the modeling or the tool you use, for example a level1 spice model of a bjt gives you a very unrealistic result, but a higher level can give you a better approximation to the reality. but an hdl simulation tool gives you a pretty good approximation for example.
so it depends on what you are simulating and on which tool you are simulating.