Laying is taking the other side, so your risk is the liability rather than the stake. Enter the lay price and stake and this shows the liability, the profit after commission, and what the position pays either way.
When you lay a selection you take the bookmaker's side of the bet: you win the backer's stake if the selection loses, but you are on the hook for the payout if it wins. That payout is your liability, and it is what you actually risk — not the lay stake. Liability is the lay stake multiplied by the price minus one: lay $70.75 at 3.20 and you risk $155.66.
Exchanges charge commission on net winnings only, so a 2% charge applies to the stake you collect when the lay wins, not to the liability. The calculator nets that off automatically and shows the profit if the selection loses and the loss if it wins, so you can size the lay to the risk you are comfortable with.
Laying is useful for trading out of a position at a better price, hedging an outright as a tournament narrows, or simply taking value on the other side of a price you think is too short.