Because German flats are often let without kitchens, the tenant leaving has a fitted kitchen they cannot easily take, and the tenant arriving needs one. The Ablöse is what settles it: you buy the kitchen, the wardrobes, sometimes the lamps and the shelves, and everyone is saved a removal.
That is the honest version and it is common. The dishonest version is a flat that is only available to someone who pays four thousand euros for a six-year-old IKEA kitchen — an entry fee dressed as a furniture sale. This matters legally: an Ablöse that is grossly out of proportion to what the goods are worth can be challenged, and the rough rule of thumb is that a price of more than about double the actual value is contestable, with the excess recoverable.
Practically, treat it as buying second-hand goods, because that is what it is. Ask what each item is, how old, and what it cost new. Look up the current price of the same kitchen. Depreciate it — a kitchen has a useful life of twenty to twenty-five years, so a ten-year-old one is worth roughly half. Get an itemised list and a receipt.
For a short sublet an Ablöse should be rare, and a request for one deserves suspicion: you are not taking the flat over, you are in it for two months, and there is nothing for you to buy. Where you will meet it legitimately is a Nachmieter handover.