As a newbie convert to carp fishing, and keen to ensure my rigs are safe for the fish, i have a question on the usage of rig tubing. 
  
Keeping things simple, while I get to grips with things,I am using a running lead generally,  without leadcore. Looking at some of the rig tying articles a lot of the rigs use tubing on the mainline. 
  
Is it to primarily protect the fish and to camolflage / pin the main line down? 
  
How long should the rig tubing be? 
  
Sorry if the question is really simplistic   
  
Awaiting replies, and thanks again