where mechanical joint is used?

mechanical coupling can be called as a mechanical device which is used to connect two mechanical links as well as transmit power between them.

mechanical joint is just used to joint two linkages . there is no power or torque transmission between them .

Sacroiliac

Mechanical joint

Lead and oakum "caulked Joint"

Welded Joint, lead wiped, lead burned, brazed, soldered, mechanical (MJ) joint, flanged joint, screwed joint, compression , flaired