Bitcoin mining uses a lot of electricity, comparable to a mid sized country. This is not a side effect: the cost of rewriting history is the cost of redoing the work, and the work is energy. Cheap to fake would mean cheap to attack.
Where the energy comes from
Miners compete on the price of electricity, so they go where it is cheapest: often places with surplus or stranded power that would otherwise be lost, such as remote hydro dams, gas that would be flared at oil wells, or solar and wind at hours when the grid cannot take it. Mining can also switch off within seconds when the grid needs the power.
The debate
Critics point out that the electricity could be used elsewhere and that some mining still runs on fossil fuels. Supporters answer that it secures a monetary network open to anyone, that it increasingly uses energy nobody else wants, and that it can help finance new power capacity. Both sides publish estimates, and the share of renewable energy is itself debated.
A useful question to ask any figure you read: does it measure the energy used, its source, or its emissions? They are three different numbers.