I have noticed an entry on a blog http://www.45n5.com which discusses a particular reason why you may want to create multiple Amazon Associates IDs for your site:
Why do you want more than one Amazon Associates ID?
The main reason to get multiple Amazon Associate ID's is for tracking. Consider each one like a google adsense channel.
Actually, this is not a great reason to create multiple Amazon Associates IDs. When you create multiple Associates IDs you are not able to combine them together and add all the products you've referred to Amazon in order to get into the next performance tier.
So, how do you then track multiple Amazon Associates campaigns you may be running? By using Amazon Tracking IDs. Unfortunately, Amazon doesn't describe how to get these tracking id's prominently on Amazon Associates Central.
The way to get these tracking id's is to email Amazon Associates Customer Service at email@example.com. Tell them the following:
1. Your main Associates ID.
2. "I would like to receive XXX Associates tracking id's to track multiple campaigns I am planning to run."
That's it. XXX can be any number up to 1,500. You will receive an email within 24 hours with your tracking id's.
Tracking id's will look very much like your Associates ID but a bit different. If your Associates ID is "MyWebSite-20", then your tracking id's look something like "MyWeb1-20" through "MyWeb1500-20".
You will then be able to use your tracking id's in the same place as you would use your Associates ID. You will get the same benefit of being able to track you campaigns separately while at the same time being able to combine all your sales together to propel yourself into the higher compensation tiers.
Per Shameek's comment below (thanks Shameek for the heads up!), if you would like to create a few tracking id's, you can do it yourself without emailing Amazon by simply logging into Amazon Associates Central web site and then clicking on:
Your account -> Manage your Tracking IDs -> Add Tracking ID
This method may be too tedious if you would like to add hundreds of tracking id's, in which case you should just email Amazon, as I suggested. However, you do have an option to name your tracking id's anything you like (with a -20 extension).