This would allow wallet operations like funding transactions with smart contract-based inputs such as LN, or any input the Core wallet doesn't know how to sign for.

https://github.com/bitcoin/bitcoin/pull/17211#issuecomment-933674310
"The inputs added will not be signed, use signrawtransactionwithkey\n"
                "or signrawtransactionwithwallet for that.\n"
                "All existing inputs must either have their previous output transaction be in the wallet\n"
                "or be in the UTXO set. Solving data must be provided for non-wallet inputs.\n"
                "Note that all inputs selected must be of standard form and P2SH scripts must be\n"
                "in the wallet using importaddress or addmultisigaddress (to calculate fees).\n"
                "You can see whether this is the case by checking the \"solvable\" field in the listunspent output.\n"
                "Only pay-to-pubkey, multisig, and P2SH versions thereof are currently supported for watch-only\n",
                {
                    {"hexstring", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The hex string of the raw transaction"},
                    {"options", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED_NAMED_ARG, "for backward compatibility: passing in a true instead of an object will result in {\"includeWatching\":true}",
                        Cat<std::vector<RPCArg>>(
                        {
                            {"add_inputs", RPCArg::Type::BOOL, RPCArg::Default{true}, "For a transaction with existing inputs, automatically include more if they are not enough."},
                            {"include_unsafe", RPCArg::Type::BOOL, RPCArg::Default{false}, "Include inputs that are not safe to spend (unconfirmed transactions from outside keys and unconfirmed replacement transactions).\n"
                                                          "Warning: the resulting transaction may become invalid if one of the unsafe inputs disappears.\n"
                                                          "If that happens, you will need to fund the transaction with different inputs and republish it."},
                            {"changeAddress", RPCArg::Type::STR, RPCArg::DefaultHint{"pool address"}, "The bitcoin address to receive the change"},
                            {"changePosition", RPCArg::Type::NUM, RPCArg::DefaultHint{"random"}, "The index of the change output"},
                            {"change_type", RPCArg::Type::STR, RPCArg::DefaultHint{"set by -changetype"}, "The output type to use. Only valid if changeAddress is not specified. Options are \"legacy\", \"p2sh-segwit\", and \"bech32\"."},
                            {"includeWatching", RPCArg::Type::BOOL, RPCArg::DefaultHint{"true for watch-only wallets, otherwise false"}, "Also select inputs which are watch only.\n"
                                                          "Only solvable inputs can be used. Watch-only destinations are solvable if the public key and/or output script was imported,\n"
                                                          "e.g. with 'importpubkey' or 'importmulti' with the 'pubkeys' or 'desc' field."},
                            {"lockUnspents", RPCArg::Type::BOOL, RPCArg::Default{false}, "Lock selected unspent outputs"},
                            {"fee_rate", RPCArg::Type::AMOUNT, RPCArg::DefaultHint{"not set, fall back to wallet fee estimation"}, "Specify a fee rate in " + CURRENCY_ATOM + "/vB."},
                            {"feeRate", RPCArg::Type::AMOUNT, RPCArg::DefaultHint{"not set, fall back to wallet fee estimation"}, "Specify a fee rate in " + CURRENCY_UNIT + "/kvB."},
                            {"subtractFeeFromOutputs", RPCArg::Type::ARR, RPCArg::Default{UniValue::VARR}, "The integers.\n"
                                                          "The fee will be equally deducted from the amount of each specified output.\n"
                                                          "Those recipients will receive less bitcoins than you enter in their corresponding amount field.\n"
                                                          "If no outputs are specified here, the sender pays the fee.",