J L I may be a year last but the difference is with a twin screw bot you can put one engine forward one reverse and cause the boat to pretty much spin on itself it’s called twin screwing and it can help you land the boat better
Obviously... It's the difference between the method of propulsion acting as the steering (as with I/O and OB) versus a fixed method of propulsion, pushing water of a rudder. You can turn an I/O, OB while the engine is idling.. try doing that with a single inboard.
