Glassfibre woven exhaust lagging is used in exhaust systems for several reasons. 

Firstly, it helps to insulate the exhaust system and reduce heat transfer to surrounding areas, which can improve safety and protect nearby components from heat damage. 

Secondly, it helps to reduce the temperature of exhaust gases before they are released into the environment, which can help to reduce the risk of fire and improve environmental safety.

Additionally, glassfibre woven exhaust lagging is durable and resistant to moisture, which makes it ideal for use in harsh marine environments. 

Finally, some glassfibre woven exhaust lagging products may also include additional features such as acoustic insulation or a reflective surface to improve heat retention, which can further improve the performance of the exhaust system.