My mom does this with her friends. She's known them all for 25 years or so, which is crucial for this. Her group is on a year-to-year commitment.
I actually asked her yesterday "why wouldn't you just put that $500 a month to the side and after a year, you'd still have the same $6k?"
What she explained to me is that for her, because she's not in a hurry to recoup the $6k that she ultimately puts in over the course of the year, she's actually helping out her friends.
So let's say there's 12 people who all put in $500 a month starting January 1st. Each month, one of the 12 people in her group gets a lump sum of $6k. My mom says she usually waits until like November (her birthday month) or December (when she typically travels for the holidays) to have it be her turn to take the $6k lump sum. Aka she's basically just getting what she's already put in.
However, for others... let's say by March- they may have an unexpected home repair or a funeral to pay for or some other life event/emergency... they can take the $6k in March, after only having put in $1,500 and use the money for whatever they need it for at that particular time.