This page explains why users need sUSD and other Synths to interact with the Lyra Protocol
Why do I need sUSD?
Lyra uses sUSD as its "quote asset", which means that option prices are quoted in terms of sUSD. Whenever you see an option price displayed in the trade form, you will see it in units of sUSD. So, whenever you purchase an option, you need to pay in sUSD. This happens when you are either:
Opening a long position
Closing a short position
In both of these situations, you are buying options from Lyra and need to send sUSD as payment.
When you open a short call, you need to lock collateral. This is to ensure that, if the option expires in-the-money (ITM) then Lyra can exercise and receive the payout. All options are fully collateralised, which means for every option sold, you need to lock 1 unit of the base asset.