It's best to have a quarantine tank. Not al fish can stand a freshwater dip. An the dip can kill them or make them ill enough to be susceptible to disease.
A quarantine tank doesn't have to be set up all the time and you will keep all your other fish from getting sick too.
A FW dip will not cure or keep any disease out of your tank. They are mostly beneficial for providing fish with temporary relief (of certain diseases) until a proper treatment can be done. FW dip is also useful for identifying a fish with gill flukes. As mentioned above, not all fish respond well to a FW dip or even a formalin bath for that matter - so IMO both should be used sparingly or only when absolutely necessary.