Google is adding additional security metadata on top of the APKs to verify that the APK was distributed via Google Play. For developers involved in countries where peer-to-peer sharing is popular, this means the app becomes eligible for updates via Google Play. This also means that Play-approved distribution channels will be able to obtain app authenticity when the users comes back online.
With phones costing more than a laptop nowadays, I don't use any apps on my Android phone that do not come from Google Play.
I was doing a real estate deal last year and my agent kept urging me to install their nifty new app to keep abreast of how the escrow was progressing. I had to pass since the app was not a Google Play endorsed app.