From Reddit: NO level sync when entering a dungeon with less that required players!

[Yimiko]

Awesome, thank you! I know in a certain other MMO with several expansions worth of old content, a favorite pastime of mine was going back through old content for cool armor sets. I appreciate the lack of item level sync. It makes for a non-stressful enjoyable trip down memory lane getting some awesome vanity pieces along the way.

For fun. S&G. nostalgia. It can be enjoyable to be challenged by something you've beaten to death once in a while. Like it could be a lot of fun to go into it as an underwhelming dps and healer duo to prove you can handle it o.o
The main point is not that you must, but that you can, more freedom is better.