It's to the point where I feel like I'm getting robbed if I don't find one! And I feel sorry for all of you who've been shopping without them.
Promotional codes are like the online version of a sale. Companies give them out to help promote what they're offering. The problem is that only an exclusive group of people are told about them so it's very hard to get your hands on one. But that's no longer the case. Sites like Bargn let people find and share promotional codes & coupons. People that use these services are kind enough to post those exclusive promo codes that they get with the hope that someone else will do the same for them. It's kind of like an "I'll scratch your back and you scratch mine" system.
For example, I recently purchased a domain from godaddy.com with a promotional code that saved me 50% off the listed price.
From now on make sure you search for promotional codes before you finish checking out otherwise you'll kick yourself for not doing it!