There's nothing unique about junior SREs. They're replaceable with almost any "vanilla Dev". You don't really build a unique experience or expertise until Senior+.
While the SRE will follow the problem to the next layer or component and try understand what is going wrong there even though they never heard of the system 10 minutes ago.
[1] "I know I'm on call for this team but the guy who sits next to me wrote that code" is something I've heard way too many times.
My point is that @pluc comments suggests too specific and I wanted that his examples break down in different levels of expertise.
Nothing I mentioned deals with disposability of engineers.