Why is docking a cruise ship in a busy port so challenging? [closed]

While watching a cruise ship on the port, I then saw some smaller ships around the big ship and guessed they were helping the cruise to dock properly but i was not sure. This got me interested in how a large cruise ships enters and docks to a port. My question is what are the challenges faced by the big cruise ship to enter a busy port and then handle its huge weight and momentum to avoid any accident? Were the smaller ships I saw really helping the big ship but how can they? I mean they are so small like mouse in front of dinosaur. What was the purpose of using small ships and why can cruise not handle on its own?