Tin is a soft metal that adheres well to copper.
Dipping a properly prepared clean stranded copper wire into molten tin will agglomerate all the strands. No strands will be loose to twist out of the wire entry. Loose strands could cause a short circuit or reduce the available current carrying capacity.
Tin has a low contact electrical resistance because it is soft. Tin is so soft that moderate contact forces tend to squeeze it so that the microscopic contact points spread into each other. The effective contact surface area is thus increased.
In the case of the high contact forces occurring in screw tightened connections tinned wires or bare wires would have the same contact resistance.
UL 1059 and CSA 22.2 no. 158 terminal block standards require that bare copper wires be used during testing.
