This doesn't sound terribly different from eg Alaska's Permanent Fund payouts? The main difference seems to be that PF payments fluctuate a lot, while the Marshalls plan seems to be a fixed, predictable sum.
The US has a Compact of Free Assocation (COFA) with the Marshall Islands, which provides a lot of the funds for this. Versus Alaska where it is paid for by natural resource extraction. All that said, it is the government of the Marshall Islands making the decision to pay it out this way.
From the US point of view, this is a good program because it incentivizes people from the Marshall Islands to stay put rather than emigrate to the US mainland (which they have a right to do under the COFA).
Why is this a good thing? Naively, as an American citizen, I would rather have someone emigrate and work in America where they can provide additional labor to the mainland economy