What is the advantage of circular DNA in bacteria?
Vibrio cholerae is known to have two circular chromosomes.

Bacteria cell division is a lot simpler and efficient as compared to eukaryotic cell division, partly due in part to the nature of their chromosomes. They don't have to undergo mitosis -- condensation of chromosomes, segregation, spindle fibre formation, attachment et al aren't involved in bacterial cell division.

Circular DNA also circumvents the Hayflick limit (thus allowing it to be "immortal"), which is the number of times a cell population can divide before it stops, presumably due to the shortening of telomeres, the sequences at the end of the chromosomes. Since circular DNA lacks telomeres, it does not get shorter with each replication cycle.

Circular DNA can also facilitate horizontal gene transfer such as Hfr mediated conjugation. Remember, conjugation is analogous to a "rolling-circle" type replication which is of course, only possible on circular pieces of DNA.