The use keyword tell the compiler that the script being compiled "uses" the specified script's classes, procedures, and/or variables (if script.000).
When a script is compiled, an object file is created. Object files contain information about the script. If other scripts need to use the classes, procedures, and/or variables of it, they use the "use" keyword, and the object file is read.
For example, if you compiled a file myscript.sc, myscript.sco (the ojbect file) would be created. You could then use that script in other scripts. Below is an example...