• Background color of a cell
  • Format cell as percent
In a complex document it's useful to color-code cells according to what kind of data they represent (monthly costs, yearly costs, totals, percentages, years, ...).
Showing numbers as a percent is helpful when doing financial modeling (interest rates, ...).
Bonus points: format cell as dollars, dates, etc
Formatting buttons probably belong in the toolbar, not in each cell, to allow them to be used on a selection of multiple cells.