Design management is all about continuity and detail. It ensures that brand colours are as consistent as possible across all media, that the correct secondary typeface is being used, that the right version of the brandmark is used in black and white - not just a greyscale version of the colour one. Seems pretty obvious, but surprisingly difficult to get some people to understand.
Your brandmark will have been a big investment in time and resources. As your business grows and more people get to handle the brand, you have to ring-fence it from well meaning, but damaging, interpretation. Which is where the design manual comes in very handy. A digital design manual, in the form of a CD or even as a section on your web site, provides a central resource for all instances of your brand and brand related material and ensures that anyone working with your brand, in the future, will understand how to use it. Unless you are only ever going to work with us of course, well why wouldn't you?