What do plasmids often contain that is helpful for bacterial survival?
Plasmids are circular dsDNA fragments in bacterial protoplasm which replicate independently of bacterial DNA.
Plasmids often acquire certain mutations which causes them to produce certain proteins which may help the bacterium to resist bacteriophages or antibiotic or other abiotic factors like radiation, temperature etc.
These are not a problem when they are in a single bacterium.But if this bacterium multiplies or transfers this plasmid to other bacterium after death, then it can cause the resistant plasmid to spread.
THere are many bacteria which have acquired resistance against antibiotics due to their overuse and it is spreading throughout the world.They acquire this when during an infection, certain bacteria which are naturally immune to antibiotics survive and pass on this resistant plasmid to other bacterium.