Why's that? Because it's robbing the pump or making it work harder?
I definitely may be incorrect but my thinking is wouldn't I want to protect the pump before all the debris got to the pump?
Making it work harder. You want to have a coarse filter (as in the strainer on your sending unit) installed before the fuel pump and a fine filter installed between your fuel pump and your carburetor. Its a lot easier for a fuel pump to push fuel through a tight place than it is for it to pull fuel from a tight place. This is especially true for electric fuel pumps, I would imagine mechanical fuel pumps are more forgiving.
What you need to do has already been mentioned...clean your gas tank out, replace your strainer on the sending unit, replace your rubber lines and flush out the metal hard lines. I would think you'd probably be better off just replacing the hard lines IMO
