Some schools will need supply teachers to cover on an ad hoc basis, such as when a full time teacher is off sick. Others will need a supply teacher on a longer term basis and may choose to work with the same supply teacher over the course of a whole term. This can give job security to the supply teacher, so is a main benefit for them, whilst also providing consistency for the children who they will be teaching.
Supply teachers will often be happy to commit to longer term positions in a school, whilst still remaining flexible by choosing this form of employment. Even longer-term supply teaching posts are still not permanent and so the teacher has the flexibility to explore more options should they choose to. Longer term supply teaching can look very good on a CV and show that a teacher is reliable, while still offering valuable flexibility.