So what's the difference between doing this and saving money in a bank or putting it in a mattress? First, there's the chance that you might get the lump payout before the year is out. That's an attractive feature. Second: well, read the complaint in that link: "So, the main reason for paying into a Susu is that the members lack fiscal discipline, and spend whatever money they have on their hands."
That's not a nice way of describing the situation. True, many people are no good at planning for the future. But in Liberia, and many other places, it isn't just you who determines how your paycheck is spent. And family obligations are extremely elastic. If you have $10 extra, your third cousin will explain to you that his child needs school fees. It is very bad form to stiff your family. But if the money is in a susu, it is out of your hands. When the $120 finally comes around to you--well, that's enough to replace the roof, which is what you needed the money for in the first place.