A function can be useful in creating large scale software systems because, if defined correctly, it presents a useful abstraction for the developer.$$ This is useful for the developer because $$

the developer only needs to know what actions the function performs, not how it performs them

the function can be used as a building block in creating a large system and helps the architect$/$designer manage the system's complexity

using pre$-$built functions fosters code reuse and saves time and money

all the above