Can you give a brief example of the situation you are describing? Or a link to an article on this?

I am really interested in improving my coding using functional programming, composition, etc... but so far, I haven't been able to understand the benefits in my current circumstances. (I've done a lot of reading already) I am hoping I will run into one, so I can see clearly why it's worth the effort to implement. (ie, implement something new when upgrading/replacing some old OOP code...)