The most "new" and technical reason to build a Pillager Bay is for a Raid Farm . By building your outpost over a bay, you can use water channels to push Ravagers into a kill chamber. This is currently the most efficient way to get Totems of Undying and Redstone in 1.20.
Summary
Unlike the official Village & Pillage update that introduced outposts, The Pillager Bay is a fan-run community primarily hosted on platforms like Telegram. It identifies itself as a pioneer in "pirating" the Minecraft Marketplace, offering "keys" and direct downloads for DLCs that would otherwise cost Minecoins. Key Features of the Community the pillager bay minecraft new
If you want to find it yourself (Java Edition 1.21+): The most "new" and technical reason to build