Having primers with a Tm between 58C and 66C provides an ideal range for annealing during the thermocycle process used. If the temperature is higher, primers will bind more specifically. However, if the temperature is too high, the primers may not be able to bind at all, resulting in no amplicon produced. If the melting temperature of a primer is too low, it is likely the primers will not be able to bind specifically. This will cause the amplification of non-specific products, leading to many different amplicons of varying lengths. This is not ideal, as it is more challenging / not possible to extract a single product from a PCR reaction with an annealing temperature that is too low (by band excision).