There are three main reasons for adding campden tablets to your brew:
1. To remove chloromines from your tap water. This is only really needed for fermentations using a large portion of tap water, i.e. beer. For beer this is added before the mash so that any sulphur dioxide that has been generated will blow off during the boil.
2. To inhibit wild yeasts and bacteria in the wort/must before pitching your own yeast. Most commercial yeasts are more tolerant of sulphites than wild yeasts but it is still a good idea to delay pitching your yeast by 12 hours after adding a campden tablet. For beer you do not need to inhibit wild yeasts because you've boiled most of them to death anyway. This is only needed for wine/cider using raw fruit.
3. To prevent oxidation and infection after a fermentation. Campden tablets will scavenge oxygen that is dissolved in a liquid. A campden tablet can be added before racking or bulk aging (though maybe not directly before bottling if you intend on drinking it straight away). This works well for wine and cider, but for beer:
The thing is that campden tablets on their own aren't the magic bullet they are sometimes made out to be. For wine and cider it's not just the campden that is preventing the infection and oxidation, it's the acidity of the liquids. So for beer with much less acidity, campden is not particularly effective for usage 2 and 3 above. Maybe it has a use in a kettle sour!
