Numbers used in equations make the equation more difficult to understand. The reader may ask “why this particular number?”, or “what does this number mean?”, or “where did this number come from?”. There is no way, short of a cell comment, to explain the number. whereas, a number in a cell may have a textual description written in the cell next to it, e.g. “This number is the tax rate for 2015.”
The few exceptions are the control arguments for certain functions, usually TRUE /FALSE values. For example, the ‘lookup type’ argument for the VLOOKUP() function or the ‘match type’ for the MATCH() function may be written directly in the function.