Different materials are used in mig tips for resistance to predominately heat.
High heat wears tips faster and causes extreme feedability problems.
Different wires are designed for different metal to be welded. For instance hard facing wires are for putting a wear surface on metal. Wires are used in different diameters for different metal thicknesses and deposition rates needed. Metal cored and flux cored wires have higher deposition rates than solid wires for instance.