1st reason would be because an email will convert a lot better than a blog post, an email is much more personal than a blog article. When someone gives you their contact information, it shows that they trust you and they're willing to build a relationship or connect with you.
2nd, Building an email list allows you to segment your subscribers in 3 sections: lurkers, freebie seekers and buyers. Lurkers are mostly those who don't participate or engage, they just follow your content maybe via social media and that's it. They do look around/browse but they never engage.
Then you have freebie seekers, these are people who optin to your email list, they follow you and they comment especially when you give away something for free. They open your emails hoping to get more free stuff and maybe one day they'll buy something. I've seen people buy after being on the list for 200+ days.
Finally, you have your buyers. These are people who buy your products and services. So when you have an email list, you can segments your list into these 3 categories and focus your energy on the most important people. Even if you're an affiliate and not selling your own products, you can giveaway bonuses and make people optin to a new list to get the bonus material or move them to a new list manually.
3rd, with email you're able to track your progress. You can check how many people opened the email, number of clicks, conversions and most importantly, WHO. Which again can make it easy to move inactive subscribers to a different list and try to get them to engage or simply remove them, this will also lower your cost for the autoresponder.
4th, you can automate the whole process. Once you've split tested your funnel and are getting good conversions then you can simply let it run on autopilot.
Those are just 4 reasons among many other reasons like getting to know your demographics, which country your product is selling the most, know your loyal customers etc.