What is the point of a Haveno security deposit in case of a irreversible payment methods?

Placing an offer to sell my BTC and get XMR, it prefill 15% security deposit for both traders and this % is customizable. Why not to set it to 0% if both payment methods (BTC, XMR) are irreversible and why Haveno-reto does not set it like that automatically?

Existing offers of this currency pair usually has 15% or 40% deposit and the fee 1.8-20% which is also weird, I would expect people would be willing to trade like 0-1% just to cover the crypto. fees.