I have USD or EUR. I want BTC or ETH. To sell my USD or EUR in exchange for BTC or ETH, I use a centralised exchange. Then I immediately transfer the BTC or ETH

I have USD or EUR. I want BTC or ETH. To sell my USD or EUR in exchange for BTC or ETH, I use a centralised exchange. Then I immediately transfer the BTC or ETH that I bought to a wallet that I control. Centralised exchanges are useful.