Non-absorbable sutures by resisting the body’s attempt to dissolve it stays inside the body without getting absorbed which may require removal by a surgeon or left for permanent wound support in the place of implantation.
Non-absorbable sutures are basically used in places where permanent or long term wound support is required or at the places where suture can be removed post healing of the wound.
For example post completion of ventral hernia surgery and implantation hernia mesh, long-term support is required to keep the polypropylene mesh in place. During such long term loan support non-absorbable sutures are used.
Non-absorbable sutures are resistant to body fluids and therefore they do not get degraded at all (Polypropylene and Polyester) or gets degraded very slowly over a long period of time but does not get absorbed (Nylon) or gets degraded and absorbed over a long period, say in two years (Silk).