Basically, the idea of adding campden tablets is the modern equivalent of burning a sulphur wick inside a barrel in order to kill off any nasties.
Each tablet is a dose of 50ppm per gallon of potassium metabisulphite (or you can just add sodium or potassium metabisulphite powder). It fills a multitude of roles, sanitising, killing off random unwanted yeasts and pathogens on equipment or in the juice, binding with O2 to reduce the opportunity of oxidation etc. It dissipates quite quickly after doing its job.
Although a tablet can be difficult to crush and dissolve, it is a convenient method which avoids having to use powder or a solution which is quite corrosive (I make up a stocksolution made from powder).
A "normal" dose is 50ppm but is somewhat dependent on the acid level of the juice (less acidic = more sulphite needed... if you want to be very precise there are tables for this). As a matter of routine, I will add an initial 50ppm dose to my juice 24 hours before adding the yeast of choice, as "insurance just to clean things up" since usually I am using juice pressed from my apples (which can have yeasts, pathogens, etc that I don't want).
However, it probably isn't essential given that most equipment is glass, stainless steel etc, well cleaned and water supplies are pretty healthy these days. Besides, some people claim to be sensitive to sulphites although I haven't had that problem with my cider.
It is a matter of balancing the issues and making your own decision.