A similar rule is often taught in computer programming: A constant should only appear once in program. If you use the constant PI a lot, but carelessly write it as 3.14159 in some places and 3.14159265 in other places, you could end up with inconsistencies in your computations. They they’ll wonder why that Venus probe missed the planet by 50000 miles.
Instead, define a variable or constant like PI := 3.14159265
just once in your code, to the number of decimal places you think you need, and refer to PI through the code thereafter. If you later find you need a few more digits to get the precision you need, you just need to fix it in one place.
Many modern programming languages come with pre-defined libraries of hundreds of commonly-used constants like this.