There are specials and bundles I offer through my site that would not be possible if I did not invest in my domain name and hosting.
Listeners like sales. Sometimes it gives people an excuse to invest in your music. You keep your profit margins higher by having your own site as well. If I sell my music on iTunes, someone is digging into my pockets because it's their platform.
And they should be! I'm not knocking iTunes for taking a percentage. I'm saying, if you can sell it through your site, why would you not?
Not to mention, I've sold hundreds and hundreds of copies of my book without even putting it on Amazon (they take a generous cut). I'm not saying I wouldn't eventually publish it on Amazon, but if it's doing fine with me keeping most of the money with no extra middle man, more power to it.
You've may have noticed there are ads on my site as well (extra revenue).
I also opened up promotions to help more music artists get heard. When you control the domain and hosting, you make the decisions without seeking excess outside approval.