We use this small tubing to make a receptacle for a sliding top hook on something like a plate mount. You can get a pictorial idea of that here. The 1/8" O.D. has room for a 3/32" rod. The 5/32" has room for a 1/8" rod. Round tubing comes (3) 12" pieces per order.
Square tubing is excellent for making a 2-part mount that will maintain alignment - no swiveling here! Sold by the foot and bundled with a piece of foot-long 1/4" square bar that slip-fits perfectly into the 5/16" square tube.