Practical Example from Sablier to explain return inside if statement

Sablier.sol

pragma solidity =0.5.17;

// import statements (omitted for brievity) ...

/**
 * @title Sablier
 * @author Sablier
 * @notice Money streaming.
 */
contract Sablier is ISablier, ReentrancyGuard, CarefulMath {

    // functions and events (omitted for brievity) ....

    /**
     * @notice Returns either the delta in seconds between `block.timestamp` and `startTime` or
     *  between `stopTime` and `startTime, whichever is smaller. If `block.timestamp` is before
     *  `startTime`, it returns 0.
     * @dev Throws if the id does not point to a valid stream.
     * @param streamId The id of the stream for which to query the delta.
     * @return The time delta in seconds.
     */
    function deltaOf(uint256 streamId) public view streamExists(streamId) returns (uint256 delta) {
        Types.Stream memory stream = streams[streamId];
        if (block.timestamp <= stream.startTime) return 0;
        if (block.timestamp < stream.stopTime) return block.timestamp - stream.startTime;
        return stream.stopTime - stream.startTime;
    }
    
    // more functions definitions ...

}