Does a pilot need clearance to enter the traffic pattern?
The US AIM (Aeronautical Information Manual) says:


  When necessary, the tower controller will issue clearances or other information for aircraft to generally follow the desired flight path (traffic patterns) when flying in Class B, Class C, and Class D surface areas and the proper taxi routes when operating on the ground. If not otherwise authorized or directed by the tower, pilots of fixed-wing aircraft approaching to land must circle the airport to the left. Pilots approaching to land in a helicopter must avoid the flow of fixed-wing traffic. However, in all instances, an appropriate clearance must be received from the tower before landing.


(AIM 4-3-2-b.)


(AIM Fig 4-3-1)

So it is mandatory to receive a landing clearance, but if traffic flow doesn’t demand it, there is no need to fly the traffic pattern. The controller basically has a lot of freedom here, but the standard is to get instruction to join it, even if this means to join a long final.