Are any side areas important for story purposes

[Paradroid78]

Mopping up stuff I missed before I complete act 3. The side areas I've been to have all just been for loot. Are any significant to the story?

[a passerby]

The ones most significant to the story are the zones tied to the relationship side quests.

The next most significant locations would be Flying Manor and Renoir's Drafts. But you should probably save these for post story.

[Suntorias]

Go early if you want to have a somewhat challenging final boss. Complete the relationship quests first if you want(the Maelle one is about as long as Sirene, so expect a bit of xp gain).

Don't use the thing you got at the end of Act 2 for the final boss, the difficulty feels about right like that.

[dudeglove]

the journals you can find in act 3's side areas reveal certain things but nothing crucial you don't already know from the main plot