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That's how it works. He bought STEEM to power up = STEEM POWER.
1 STEEM = 1 STEEM POWER

But why not just buy SP directly instead of steem first?

There are multiple reasons:

  • Minimum limit of buying SP directly from (https://blocktrades.us/) = 37.0705 BTC
  • If you buy from exchanges, you can buy cheaper and in bulk (If you check now if you buy 1 BTC = 1266.649 SP in (https://blocktrades.us/) while on Poloniex you can buy 1 BTC = 1305.824 SP