Source code must somehow become machine code, ultimately. Sometimes this is handled in real-time by an interpreter, as in the case of scripting languages such as Perl or Javascript. For more complex applications, though, this work is usually handled by a compiler. Therefore, you must ensure that you have a proper compiler installed, and that you are calling the compiler properly with the correct compiler options.

In the modern world, it's crazy to write all of the code yourself. When you want to write output to the screen, you don't write code that talks directly to the monitor; you use a library that handles input and output. When you want to play audio, you don't handcode the waveforms yourself; you use audio codecs. When you compile the code, you almost always need to include libraries of one kind or another -- which means you must know which libraries you need, and you must ensure that the libraries are where the compiler expects them to be, and that the libraries are all of the right version.

In complex software projects, it's vital to keep track of dependencies. A change to code in a single library can have effects across your entire project, and might require some or all of your code to be recompiled -- and often in a particular order. Keeping track of dozens of libraries, and references to those libraries by hundreds of source files, can be an ugly business.

It's essential to know when you've introduced bugs sooner rather than later; new bugs are often easy to fix, and old bugs are often not so easy to fix. Also, it frequently happens that bugs, once fixed, creep back into code. Running basic tests on a project every time it's built can be a good way to ensure that bugs get fixed and stay fixed.

Sometimes you want to install the program you just compiled so that it can be run from anyhere on the system, and other programs or users can find it. Or sometimes you want to bundle it up into a format that allows anyone to take your executable and install it easily for themselves. You don't want to do this for every build, but when you know that your build is good, one of the important final steps is to put the executable, and all documentation, in a central location.