It is possible to buy an NFT with any currency; however, this would be recorded publicly on-chain forever for everyone to see. Transactions involving SNIP-20 tokens (like sSCRT, sUSDT, sETH, etc.) do not reveal who bought a specific NFT and therefore help to protect your identity and increase your security. This also enables a far wider variety of use cases for NFTs. You can always choose to reveal your ownership of an asset, but they begin private by default.
The use of SNIP-20 tokens on Stashh unlocks a bunch of features that would be impossible on other blockchains. For example, using SNIP-20 tokens in an auction keeps the balance of the bidder hidden so their competitors can't simply inspect the balance of the bidder and outbid them.
You can buy an asset on Stashh using the Buy With SCRT feature, however the actual transaction will be conducted using a SNIP-20 token to preserve your privacy. In this case Stashh automatically swaps the right amount of SCRT for the corresponding SNIP-20 token, then uses that to buy your asset.