If you know the private key of an address,
you own the bitcoins in that wallet.
Try your hand at finding a random address with bitcoins in it.
Get lucky
What are my chances ? Well, there are a lot of bitcoin addresses. A lot. But there are, also, many addresses in use. The chances of finding, at random, the private key of an address that is in use and has funds are almost nil. Yet, by understanding the mathematics behind the generation of Bitcoin keys, they can be chosen at random from much smaller ranges, centered around used bitcoin addresses extracted from the blockchain, thus increasing your chances by quite a few orders of magniture. You've got nothing to lose, so give it a shot or two.

*As an example, think how vanity addresses are generated.

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