Whether its wires digging in, back elastic rubbing, straps causing grooves in your shoulder it all means the bra is a poor firt or poor quality.
When a bra fits badly it creates heat by pressure being put on vulnerable breast and underarm tissue. This in turn creates abraisons and sores,in extreme cases it can block the lymphatic system.
A properly fitted bra will reduce this discomfort.
These images show how heat is generated by a badly fitting bra and how a properly fitting bra reduces this pressure and therefore is safer and more comfortable.