The idea of a build system is to automate all the steps I have described up to this point. A build system should be able to build, from upstream source code, some or all of the following:
Building from an upstream source is important for a number of reasons. It means that you have peace of mind that you can rebuild at any time, without external dependencies. It also means that you have the source code for debugging and that you can meet your license requirements to distribute that to users where necessary.
Therefore to do its job, a build system has to be able to do the following: