When spreadsheet software first became available for computers in the early 1980s, it was known as a "killer application" because people began to buy computers just so they could work with spreadsheets. Today, spreadsheet software is practically a required part of any collection of office software, and it remains as useful now as it was in the beginning.
Most people process data most easily when it is presented visually, rather than simply existing as a set of hypothetical numbers in the mind. Spreadsheets allow you to lay figures out on a grid, calculating and manipulating them visually. Often, this may result in you being able to process the information more quickly. On a spreadsheet, you may also notice errors or omissions that you would not have otherwise.
Spreadsheet software gives you the ability to enter mathematical formulas ranging from simple arithmetic to complex statistics. This is done in a simple and intuitive manner; to perform a calculation with two different cells on the spreadsheet, simply click a cell, press a key for a mathematical operator such as the "+" sign and click the second cell. This is often a much faster way to perform calculations with your data than a calculator would be.
In addition to the standard method of entering data in a spreadsheet -- typing numbers in cells -- you can also create a cell with a value generated dynamically based on other cells. For example, a cell might display the net profit on the sale of an item by combining a value in one cell with a certain percentage. Because the value displayed in the cell is based on the values in two other cells, the cell dynamically updates when you change either of the referenced cells. This allows you to test different scenarios by changing the cost of an item, or the percentage of profit. Any time you base the value of one cell on the value of other cells, the value of one cell updates automatically when the other is changed.
Spreadsheet software gives you the ability to analyze your data in ways other than simply looking at grids and lines. Most spreadsheet software can automatically create graphs and charts from your data, giving you different ways of comparing and analyzing information. These visual representations can also be printed and emailed, or exported into slide shows for presentations.