It is possible to launch IPython from any Python script, even when the standard Python interpreter runs the script. It can be useful in some occasions when you need to interact with a complex Python program at some point, and where using the IPython interpreter for the whole program is not possible or unwanted. For example, in a scientific computing context, you may want to pause the program after some automatic, computationally-intensive algorithms to look at the data, draw some plots, and so on, before resuming the program. Another possible use case is the integration of a widget in a graphical user interface to let the user interact with the Python environment through the IPython command-line interface.
The easiest way to integrate IPython in a program is to call IPython.embed() at any point in your Python program (after import IPython). You can also specify custom options, including the input/output templates in the command-line interface, the startup/exit messages,...