There are 2 main reasons.

  1. Firstly, suppliers are required by their licence conditions to offer a range of payment methods to consumers. It is a regulatory condition of their right to supply.
  2. Secondly, certain suppliers are beginning to offer an increasing range of payment options as a way of differentiating themselves through providing greater flexibility to consumers.

Because different payment methods have different cost implications for suppliers, they will normally charge different rates depending upon how you pay.