In the early days of Bitcoin, anybody could simply run a mining program from their PC or laptop. But as the network got larger and more people became interested in mining, the mining algorithm became more difficult. This is because the code for Bitcoin targets finding a new block once every 10 minutes, on average. If more miners are involved, the chances that somebody will solve the right hash quicker increases, and so the difficulty increases to restore that 10-minute goal. Now imagine if thousands, or even millions more times that mining power joins the network. That's a lot of new machines consuming energy.