Nearly everyone wants their website to make money for them and one of the models for doing this is by running ads on your website.
There are a few problems with this:
1. Space on your website is only as valuable as you have visitors to your site. If you can get a lot of traffic to your site, there are usually other ways to make money from it.
2. With most types of sites, having another company’s ads running just makes you look small and ruins your branding. The only exceptions are types of sites where advertising is traditional, such as directories / portals, news and magazine sites and blogs.
3. There’s always the chance, if you are also selling things on your site, that the ads will promote a competitor of yours!