Let's not be naive by pretending improving user security is the main purpose of that move.
replyMay or may not be. It does significantly improve security.
replyI miss data to confirm any of this does significantly improve security.
replyAnecdata: I have yet to meet someone who have been targeted by unknowingly side-loading apps.
If they really want to improve security they can work on much more impactful changes. I know people having been scammed by ads proudly delivered by instagram and google. I read about malicious apps that somehow went through official store filtering.