1. You get to develop content on the blog/website which remains more or less permanently on the site. It is a way to attract traffic through whichever means, including search engine traffic.
2. You can develop one or more gateway pages on your blog/site to point to your affiliate gateways. This is a great way to ensure the work you do once, serves you more or less permanently.
If you rely highly on search engine traffic, you get to reap the fruits of your own labour and not pass on such gains of search engine optimization, to merchant companies you are marketing for as an affiliate.
If you call it quits with one merchant company, you can simply switch the traffic you have developed over time for that page on your blog/site to another merchant company, so long as it is in the same niche.