One would CNAME www.etsy.com to a CDN as a form of delegation so that the Global Traffic Management service of that CDN (Akamai in this case) can determine the location of the DNS resolver querying for www.etsy.com and return the nearest edge from their CDN network.
For Akamai, I think domains which go through their own network are *.edgekey.net and *.edgesuite.net. In this case, the IP address returned will be owned by the CDN.
You could also CNAME to a record that eventually resolves to an IP address that you own. Such cases are purely considered for Traffic Management and you don't get 'caching network goodness' along with it. The advantage of such a setup is useful when you have presence/capacity in multiple geographic locations and just need a GTM system to hand over the nearest A record to a user who queries for www.example.com. In the case of Akamai, AFAIK all domains ending with .akadns.net are GTM-only.
Another way of attaining geographic failover is through anycasting, but that's beyond the scope of this question.
About why etsy.com points to an A record owned by them: You cannot CNAME base domain records. If you want fault tolerance on a base domain like that, you can delegate that domain to your CDN, but then you're handing over ownership, which I don't think etsy in this case would want. So, they patch things up by pointing it to one of their locations and 301'ing users to their www host to make them go through Akamai.
If 184.108.40.206 goes down, etsy.com will fail, but www.etsy.com will continue to work as long as etsy has more than one origin behind their www domain.