Keeping the fistula tract open can help prevent the trapping of pus or other infectious material in the wound.
[1] Setons can be tied tightly or loosely and with different materials, depending on the anatomical location of the fistulae and what may be medically required.
A seton might be tied loosely as a palliative measure to avoid septic and painful exacerbations, or as temporary measure before surgical excision as in fistulotomy or fistulectomy.
In some types of fistulae, a seton may be tied with more tension and tightened periodically.
This is the Kshar-Sutra method mentioned by Sushruta in ancient Indian surgical practice.