What is the point of categories in contacts

I know what the point is with other email clients (Google), but for Outlook there seems to be no place where it actually pops up. I don't see any category indication in new inbox messages from people in whatever category. I don't see how I can search a for emails from people in a category. I can't use categories like groups to quickly send everyone in the category an email. If I can do any of these then I just don't know how. But I haven't seen anywhere that categories show up outside my contacts list. I use a Mac btw.