Some are extremely large and this property makes them even harder to detect physically. In general, if you isolate a new organism and it has an unstable phenotype, it would be prudent to consider the possibility that a plasmid is harbored in the strain. Their low copy number increases the likelihood that the analysis is physiologically relevant. Multicopy plasmids can cause higher level of gene products to be made under some conditions. Such plasmids are also a good source of DNA for cloning.
Conjugation is useful as an easy and efficient way to move plasmids between strains.