Often when building even a simple financial model, unit conversion is necessary.
For example, you sell 437,911 widgets at £12.50 each. However, in your output summary sheet for senior management, do you really wish to show that the revenue earned is £5,473,888 or is it more likely that you will display this figure as £5.5m?
Most modellers will modify their number with a calculation such as
=437911*12.50/1000000. But using this approach poses the danger of making a mistake in your additions by adding £ and £millions.
By using a custom number format, rather than modifying values, you will never again add or subtract numbers of different orders of magnitude.