Item Level is a glorified key to unlock higher level duties, which are required to advance in the story. If you're not into raiding the "biggest" use for keeping up with at least the craftable gear is to make undersized content easier, if you have any reason whatsoever to do undersized content, but that's about it.

I stopped doing the weekly treadmill for the capped tomes once I realised that, and how dull the "expert" dungeon design became.