Why is including my fonts with my job important, can you just substitute your versions of the fonts?

First of all, we may not have some or all of the fonts you used. Also, fonts carry programming information within them that affects how the lines of text break and determines how the characters appear on the screen and on the page when printing. These characteristics can vary from font manufacturer to font manufacturer, so substituting our different version of a particular font (i.e. Times) may cause dramatic and undesirable changes to the way the text flows within the document and the appearance of the final output.