Colour guides are used to reference and match colour. They include hundreds of swatches with universal colour codes, which you can reference to ensure consistency across your collection.
What you see on your computer screen may not be the same as someone else’s monitor, or your printers monitor, or their printer monitor (you get the picture..). So to avoid inconsistency, you need a reliable guide. There are multiple colour guides, with the leading provider being Pantone.
Having a colour matching system in place is incredibly helpful and much easier than trying to say "a little bit more red"... you’ll never end up with what you want!