Split allows you to look at different regions of the spreadsheet at the same time. Say you want to look at cells D10:F30 and you want to compare it to something in X50:Z70.
You can then split the screen into sections and look at it in different pieces without having to zoom out really far to see both sections. I guess you may use it, if you have a formula that is maybe changing by a cell that cannot be seen on the same screen and you want to see how changing that cell might affect that formula?
It is in a sense the same as freeze panes, but you have a little more flexibility with the 2 regions.
Hope that helps.