SimpleWallet.sol

pragma solidity ^0.6.2;
pragma experimental ABIEncoderV2;

import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/cryptography/ECDSA.sol";

contract SimpleWallet {
    
    mapping(bytes32 => bool) public prevCalls; // Guarantee one-time use
    address public owner; 
    
    event Forward(bytes32 txHash, bool success);
    
    constructor(address _owner) public {
        owner = _owner;
    }
    
    function forward(address _to, bytes memory _data, uint _nonce, bytes memory _signature) public {
        bytes32 h = keccak256(abi.encode(_to, _data, _nonce, address(this))); // add chainid
        address signer = ECDSA.recover(ECDSA.toEthSignedMessageHash(h), _signature);
        
        require(signer == owner, "Only the owner, SGX enclave, can authorise calls.");
        require(!prevCalls[h], "Message was already sent");
        
        prevCalls[h] = true; // Store to prevent future use
        
        (bool success,) = _to.call(abi.encodePacked(_data));

        emit Forward(h, success);
    }
}

What's the rational behind this contract? Is it a stepping stone to a fully fledged forwarder contract?

Its just an extremely simple forwarder / wallet contract. minimal gas, storage, supports concurrent transactions, etc. if a new function that used delegatecall() was added & it was updated to send value, then it is essentially a fully fledged forwarder contract.