Why do bacteria have different plasmids?

Answer 1

The nature of the plasmids depends on the nature of the bacteria. Some bacteria may have plasmids that confer resistance to relevant antibiotics or may confer advantages specific to their niche or in regards to how they cause disease.

For example, an intracellular pathogen could contain a plasmid that encodes for INV which allows for them to invade host cells. An extracellular pathogen, on the other hand, would not necessarily need this plasmid.