Some of us like our GUIs, but some of us yearn for the control and options that only the command line can give. The creators of Slic3r are our kind of people. They've built in command line usage into Slic3r from the early stages of the project.
So far we have created prints from the graphical interface. Slic3r can also be controlled and tweaked from the command line, which gives the user more power and control over the printing process.
So now that we know about the command line, how do we get to it?