Sometimes when you are working with a big file you need to see two parts of it at once. You might be writing your results and need to look at your methods at the same time so that you put them in the same order. Or you might need to look at your introduction while writing your discussion. Or you might be reading a pdf and you need to see the reference list while you are reading. Or you might be working with big spreadsheet and you need to see the end while you are working at the beginning.
Instead of flipping back wards and forwards, you can quickly and easily show two parts at once using a little tool that hides at the top right of the scroll bar on the right side of the window. See below for screenshots with the area indicated with the arrow. When you mouse over this area your mouse changes to the split screen symbol and you can left click and drag to split the screen. Each section can be scrolled and even zoomed separately.
In Excel, there is another split screen tool in a similar position at the bottom right of the scroll bar. With this one you can split the screen vertically.
See the screen shots below for where to look.