Line graphs are often used to plot changes in data over time, such as monthly temperature changes or daily changes in stock market prices.
Graphs, in general, can make it much easier to interpret important data in a spreadsheet. They make it easy for you to draw attention to data trends and patterns in a spreadsheet that might otherwise be difficult to see.
With the Chart Wizard gone from Excel 2007, creating charts is quite a bit different in this latest version of the program. To find more about adding graphs to an Excel 2007 spreadsheet, read this step by step tutorial on creating an Excel 2007 Line Graph Tutorial.