Typically you wouldn't use lgamma to compute exact values of factorials at all. For a real world example of this function in action, look at John D. Cook's blog:

[1]: http://www.johndcook.com/blog/2012/07/14/log-gamma-differenc...

[2]: http://www.johndcook.com/blog/2010/08/16/how-to-compute-log-...

[3]: http://www.johndcook.com/blog/2008/04/24/how-to-calculate-bi...