Strange. Google states (as does Adobe) that its browser uses Adobe Flash Player plugin, and it updates it by default, as does IE.
of course. there is (less and less) Flash content on the internet. browsers nowadays use HTML5 by default, but if the content is not available as HTML5 then the plugin is needed. on YouTube it is needed for the one less capable browser (MS Internet Explorer) that does not support WEBM.
Turning off a plugin doesn't solve the problem of not being able to see content streamed only using it.
turning off a plugin to play only on demand has several benefits. most importantly it protects you from potential malicious code that would otherwise be executed as you load a website in your browser. it also saves you from blinking Flash ads in case you do not use an addblocker. and third it saves bandwidth and loads sites faster.
in a modern browser you can also define exceptions from the general rule.
btw.: that Aviator browser that you (i hope: no longer) consider to be "very secure" has this setting turned on by default if i remember correctly