Usefulness of 400?
It’s level 500 for ascended, 400 is the level for crafting exotics (and account bound legendary ‘gifts’). Ascended are the only ‘non-tradeable’ items that you can currently craft at 500, but there have been hints of precursor scavenger hunts and new legendary items which may require you to have a max craft also.
As for chef, at the moment there is only level 400 on cooking and everything you can make is tradeable, with the exception of legendary gifts. Who knows if they will venture down the route of ascended food items? Or indeed make Chef responsible for additional legendary related gifts at 400 or 500 level crafting. Your guess is as good as mine.
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All you can do with the crafting (except ascended) you can also buy it on the trading post. But it gonna cost you more on the TP than if you craft it yourself. Still getting to 400 mean a huge initial investment (crafting material, gold and time). So is it worth it?
Depend on you. If you are a casual player with 1 or 2 character, that you still play with rare gear (because you can totally do that and still enjoy the game), don’t even want to hear about legendary and don’t need that much gold because you don’t buy much stuff. Then you don’t need to max crafting.
On the other hand. I use crafting to partially level up 5 of my alts so why not maxing all profession. I use some level 400 to craft corrupted and destroyer weapons, i crafted my legendary, some stuff cost less if you craft it and don’t buy it on the TP, and finally i want my ascended weapons and in the future my ascended armor. So for me it was a good thing to max my crafting profession.