5how you actually get it
Developer outfits are locked behind treasure bags, which means an Expert or Master world and a Hardmode boss you can beat repeatedly. Every Hardmode bag can produce one except the Queen Slime bag. Pick the boss you can re-summon and drop fastest, then open bags until it appears. When it does, D-Town's Wings come out alongside the clothing, and unlike the rest of the set those are real equipment.
what the whole outfit does
Half a set gets you nothing special. With the Helmet, Breastplate and Leggings all equipped the costume lights up and pulses, borrowing the shimmer that adamantite gear has along with the glow that the spooky set puts out. It is loud on screen, which is the entire point of a developer costume.
who D-Town is
The set was built for Re-Logic's business director, one of several staff outfits scattered through the game. For a long stretch it sat in the files with no legitimate way to obtain it, and getting hold of one back then was closer to a punishment than a prize: it dragged your movement down and kept a pile of debuffs stapled to you until you unequipped it. Both problems were removed when the drop was switched on.