Adsense Advantages

AdSense is simple to join.

It's easy to paste a bit of code into your pages.
It's free to join.

You don't have to spend time finding advertisers.
Google provides well written, highly relevant ads - chosen to closely match the content on your pages.

You don't have to waste time choosing different ads for different pages.
You don't have to mess around with different code for various affiliate programs.
You're free to concentrate on providing good content and Google does the work of finding the best ads for your pages from 100,000 AdWords advertisers.

It's suitable for beginners or marketing veterans.
AdSense provides simple, easy-to-understand stats.

If you have affiliate links on your site, you ARE allowed to add AdSense ads. However, with your affiliate links, you must not mimic the look and feel of the Google ads.
You can filter up to 200 URLs, so you can block ads for sites that don't meet your standards. You can also block strong competitors.

Inevitably, AdSense is competing strongly for space on web sites with all other revenue sharing opportunities.

If you own a small web site you can plug a bit of AdSense code into your site and almost instantly relevant text ads that are likely to appeal to your visitors will appear on your pages.
If you own several sites, you need apply only once. This makes AdSense much simpler than joining a bunch of affiliate programs.