The “shiny” weapons are rare versions of the BRAVE weapons. They are a limited-time drop with double perks options, they come fully masterworked, and have a limited-time ornament that can drop in place of any drop of a BRAVE weapon you earn. You receive an example of each when you complete the weapon quests from Arcite, although these are set rolls that can be hit or miss. Most of the reason to grind them is to flex as the ornament will not be available after Final Shape launches: double perks can be handy but the odds you get the roll you want on a shiny is already low, nevermind getting the roll you want and some extra good perks to swap to. In a practical sense, they offer little to no advantage (barring a few extreme exceptions that will likely be nerfed or made completely unexploitable in the near future) so whether they sound worth it to you is mostly just if you think limited time cosmetics are cool.