Been down this road.....
At least in the one I looked at, the buy-in option was only those who wanted to start in what was considered Independent Living.
You sell your house and "buy" a condo/townhouse on the property. You live on site, have access to all the meals, programs, etc., for a monthly fee, but aren't using the care services. Basically an all inclusive retirement community. Cleaning, lawncare, maintenance etc. is all taken care of. Simplifies life, but you still have your own place, own car, and full independence.
If and when you need assisted living, you either a) stay in that townhouse/condo and pay an increased fee for help to come in, or b) move out of the townhouse/condo and into the actual facility and live in a pod with assistance on hand (higher monthly fee)
Then you would transition from there to skilled nursing / nursing home care in a hospital room setting if it progresses that far.
The Independent Living option is for those who want to have their own place, don't need any care, but want the convenience of all the services. They're the ones who "buy-in" and then resell at some guaranteed price point.
Those who come to the facility directly into assisted living and or skilled care don't do the buy-in / re-sell, and simply pay the monthly cost of whatever care level they require.
The buy in / buy out is sort of like a required put option on the condo when you move, die, or go into the main facility for assisted and skilled nursing.
I know a widower in his early 70's who did this. Sold his house, got an Independent Living Condo and knows he won't ever have to move again. Still travels a bunch, and when he is gone he doesn't have to worry about anything. Zero maintenance, doesn't have to worry about a house, maid, lawn guy when he's not there. When he's there, he's free to do whatever he wants, has everything he needs, none of the ownership hassles, and uses the amenities as he choses.