The advantage of fibreglass is that it could be easily be moulded into any shape, has mechanical strength that is so strong and stiff for its weight that it can out-perform most of the other materials.
Fibreglass last a long time, it can be coloured, shiny or dull. It is low maintenance, anti-magnetic, fire resistant, good electrical insulator and weatherproof.
The disadvantages is that it needs to be re-gel coated about every five years and can result in airborne fibres which may be an issue to asthma sufferers. Required Gaps around down lights and electric boxes reduces the effectiveness, loses effectiveness if Insulation becomes wet or damp and irritation during installation.