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Why should someone need to buy WBTC if you can buy BTC with the same price?
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I am not sure if this real question, but obviously because people want to use decentralized exchanged to trade it. People who are asking why anyone should use CEX could be asking you that question instead. It was a way to trade cryptos with different blockchains against each other. There's really no perfect system for doing that, as every way has their own downsides.
Did you mean people who want to trade their BTC in decentralized exchange must use WBTC and if they want to trade on centralized exchange they must use BTC? This is really a big misconception since WBTC that traded on Uniswap, Pancakeswap etc aren't really 100% decentralized! Actually the best decentralized exchange is Bisq and you must use BTC to use it, they didn't list centralized scam coin e.g. WBTC. Bisq offer decentralized exchange with high anonymity since you use Tor network.
