With intensive farming systems worms can often be a source of signifigant disease, causing reduced weight gain, scouring and loss of condition. In severe cases they can also cause death.
Anthelmintics (wormers) are expensive drugs and so they should be used strategically and not just as a matter of routine. This situation is further complicated by known resistance of worms to certain products which can render them totally ineffective.
For these reasons it is important to undertake a carefully prepared worming strategy which inevitably will be unique to each farm. Factors to be considered would be available grazing (permanent or otherwise), previous grazing history, stock numbers/breed and ideally monitoring of faecal (dung) samples. All of these will also be affected by seasonal influences along with temperature, rainfull and humidity. Organic production units can also only use anthelmintics in accordance with their production authority ie "Soil Associatiion" etc.
Worming strategies are usually included in flock or herd health plans and should be reviewed periodically.