Campaign badges are a long established military tradition and are often included in the fluff of 40k. They are a unique opportunity for the hobbiest to be creative and to proudly brag about the games they've played. After all how many of you have played against someone who has medusa V or even the 13th black crusade written on a banner. Campaign badges mark your force as veterans. They allow an opening to talk to fellow gamers of the campaigns you've played with friends. They even allow good natured taunting and rivalries between a groups members. And they are a chance to truly distinguish your models from others. Campaign markings can be complex like those seen above or simple like the inclusion of a stripe or lightning bolt to a shoulder pad. All these things allow your models to be unique to you.
For example- say you had a memorable game vs orks playing the relic mission. The inclusion of a grail cup atop a pile of ork teeth might make a nice campaign badge. Another example would be a lowly guardsman managed to kill a imperial knight with a melta gun. A broken knight skull image or even a simple black executioners axe emblem could make a great army badge.
Their are many places one can look for inspiration of a campaign badge from heraldry charts, historical images, to even forge world books as you can see below.
Adding a campaign badge can be an army wide project or limited to a few individuals in your army. if you have a particular unit that has repeatedly done well you can chose to paint it in a variant scheme rather than a badge. A great example of this would be a terminator unit that survives and does spectacularly good in multiple games. Changing the scheme slightly to reflect the elite status of the unit is both unique and fun. For examples of what I mean, I suggest you check out the Imperial Fist terminators below.
Remember do what you want to make the models your own and have fun with it and you'll never have cause to fully regret it.