Agencies usually build their stuff for big companies in a separate domain and as a campaign. Via this method, companies like Spotify can hold extremely strict control over their domain and web contents.

Furthermore, the agency (which now holds strict control over its new domain) can use its very own methods for improving things like page speed, infrastructure (AWS, App Engine, Azure or else)... At this point you could say: "and what about a sub-domain?" Well, an answer for this one could be more aligned with security reasons and —let me introduce another reason— with tracking (Analytics, campaign) reasons.

Agencies also hire a lot of front-end people to do cutting-edge things like the ones you see in spotify-yearinmusic.com. Plus, they do this everyday. Meanwhile, Spotify developers core operations probably focus on "big problems" or the "big picture" such as the back-end (which btw is vital for Spotify's economic success).

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In spite all the above, I totally prefer leaner URLs like spotify.com/year-in-music/2014 and I think there's no valid reason for avoiding this beautiful, UX-oriented, less confusing practice.

Truth is: as companies (organizations) get bigger, big people (the ones with the UX, perfectionism culture right at their DNA) can't be involved in every little decision made at the company.

Now, link this last thought to Facebook. See: • facebookgroups.com uses cloudfront.net for assets. • facebook.com uses akamaihd.net for assets.

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I'm already exhausted for comparing the code behind spotify-yearinmusic.com with the one under spotify.com. This means you've got homework :)