It's called comparative advantage. Have junior engineers work on easy things that are a waste of time for senior engineers to do. Personally, I'd rather not have my $200,000 a year senior engineer working on debugging cross browser javascript or writing documentation.
Hopefully he's not wasting a bunch of time debugging cross-browser javascript because A) He already knows how to fix it, or B) It didn't break to begin with. JavaScript is hardly difficult material.
As for documentation, I'm not sure quite what to say. The best person to document code is the person that's writing it. The best person to document things that aren't code is a technical writer, not an engineer.
Those were just example of simple tasks that a junior developer can do. And a junior develop in my company is someone of any age who has less experience. What kind of industry would we have if we only hired senior developers? If we said junior developers have no value? I guess we'd be a bit like the fashion industry where anyone new is just supposed to work for free.
