Tugs are used when a ship can't maneuver on its own because it has to go too slow. This is usually because a ship needs to port and must turn around and such. Without propulsion pods that can turn to push sideways and such like cruise ships often have, tugs are necessary. Ships like this must be powered and moving through the water in order to turn. They only have one or two big propellers at the back so water must be moving past the rudder in order for them to turn. Once the ship was moving out to sea under its own power, they would have disconnected the tugs. (my words are just not flowing today, but hopefully you can understand)
Tugs are used when a ship can't maneuver on its own because it has to go too slow. This is usually because a ship needs to port and must turn around and such. Without propulsion pods that can turn to push sideways and such like cruise ships often have, tugs are necessary. Ships like this must be powered and moving through the water in order to turn. They only have one or two big propellers at the back so water must be moving past the rudder in order for them to turn. Once the ship was moving out to sea under its own power, they would have disconnected the tugs. (my words are just not flowing today, but hopefully you can understand)