Formatting cells in Excel: Borders

What are borders useful for?

If you apply a background color to cells in the worksheet, the gridlines on those areas will disappear. This might make the data seem unconnected and less readable.

Moreover, did you ever try to print out your data? The gridlines don’t appear when printing, making your data float in open space on your paper sheet.
Selecting the data area and applying a thin border line within it, will solve these issues.

Drawing wide lines can help you distinguish between areas in your data. For example, if you have a table with the different months on the first title row, you can draw for every three months a wide border line, making the year quarters (Q1, Q2 etc.) more distinguished.

If you are using totals calculations on the bottom row of a table, you can draw a double line above that row, to separate it from the data.