This corrects the angle of the brake pads relative to the rim, the cable tension, and the spring tension in the brake.
This corrects the cable tension to the derailleurs and the derailleur stops (to prevent the chain from jumping off.
When a wheel becomes slightly bent, this can be corrected by increasing and decreasing spoke tension until the wheel spins straight.
Bearings are located in the headset, hubs, bottom bracket, and pedals. When bearings become loose the corresponding parts will rattle around. This can be corrected by tightening the cones and lock nuts.
In a bearing repack the bearing assembly is completely disassembled. All of the components are cleaned, and then reassembled with fresh grease.
Note: Many new performance bikes use sealed bearings that do not require repacking.