A coupler is a very short pipe or tube that has a socket at one (or both) ends. This allows two pipes or tubes to be joined (or coupled), welded or soldered together.
An adapter is used to convert the attributes of one device to another. For example, an electrical adapter will enable connections between sockets used in one region of the world with another.
One example of a need for a coupler is the shaft coupler. Shaft coupling can deliver and disconnect power. This helps separate machines, improve flexibility, and reduce shock and overload.
Ball valves and pressure gauges often require accessories to function properly. Universal joints will allow movement in ridged rods and for rotary motion to be transmitted.