Cashtab used `wif` for private keys before the ecash-lib implementation. After ecash-lib, it uses hex. The wallet storage already keeps the format used by ecash-lib. So, there is no need to convert the wif to get this sk.
This is an artifact from before the wallet migration, when we only had wif stored, but we needed sk to build this specific kind of tx using ecash-agora.
After this, we could stop storing and supporting wif, tho this would require a light wallet migration (just stop storing this key). Mb we want to keep wif support tho, as it could be useful if we ever want to support importing paper wallets.