Suppose you want to draw a six sided star. You could do it by drawing a triangle and then invert that triangle and draw it again. But suppose you want to cache

Suppose you want to draw a six sided star. You could do it by drawing a triangle and then invert that triangle and draw it again. But suppose you want to cache the two triangles so that the next time you draw it will happen more quickly. You have some "Triangle invert(Triangle t)" function. It takes a triangle and returns the inverted version of that triangle. If that function modifies the triangle in place then you will first have to make a copy of it to make sure that you have the original triangle. If the code is written in a functional language you can assume that the funtion will always return a copy. If it is not functional then you may not know whether the function returns a copy or not and it may come as a "surprise" that the function modified the triangle in place. In theory the functional approach is more stable and predictable. But whether this really causes a lot of bugs in practice is another question.