WWDC is a conference for developers, but Apple's developers are almost all
B2C developers, so the keynote has a specific purpose related to that.
The keynote is basically there to prove to Apple's B2C developers that the "Apple platform" will still be a viable market to sell to for the next year, such that it's worth investing time into to learn all their proprietary platform stuff at the very same conference.
Apple proves their market-viability going forward, by demonstrating all the consumer-facing features that will attract consumers to their devices, where they can then spend money on the B2C developers' apps.
This is also why they screen ads: they're showing the developers their strategy for attracting customers, to buoy up their faith in Apple's market strategy. Apple creating impressive ad copy for their platform directly affects your bottom line as an Apple B2C app dev—so you want to see that they've done it.
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Note that all this stands in stark contrast to Microsoft's strategy: they love their developers too, but those mainly consist of B2B developers. And you don't win over B2B devs by proving that your platform will attract home users. You win over B2B devs by showing all the Enterprise-y features which your platform will use to guarantee the Enterprise-y deployments the B2B devs sell to; and how it will easily integrate with everything customers already have, such that there won't be an "incompatible" ecosystem of boxen serving some purpose that grows on its own rather than being absorbed into a larger Active Directory domain.
This is, for example, why Windows Server 2016 is all about virtualization: the latest incompatible-cluster tumor growing in the middle of what are otherwise Microsoft shop is private "virtual cloud" infrastructure using VMWare/Citrix/RedHat VM host appliances. If Windows can't integrate with those, acting as their peer or manager or SAN or whatever, then those machines will take over the datacenter and Windows will just become the thing being run in a VM on them.)