Rockwool has many advantages
It has a large water retention capacity, and also holds sufficient air. It holds a lot of water which gives you an advantage against power or equipment failures. And it also holds 18 - 25% air which gives the root system ample oxygen so long as the media is not completely submersed.
It is available in multiple sizes and shapes for various hydroponic applications. Everything from 1" cubes to huge slabs.
Rockwool is also durable, it doesn't readily fall apart or break down.
Rockwool also has some serious disadvantages
That same quality that makes it durable also makes it an environmental nuisance. Thrown away it does not biodegrade - It's rock. A million years from now it will still be sitting where ever it was disposed of.
It is also a health hazard. Rockwool, like asbestos is made from rocks and minerals the fibers and dust thereof have proven to be extremely hazardous. Wearing a dusk mask when using it is advisable. Rockwool is not as serious a threat as asbestos, but it is still unhealthy.
Rockwool has a high pH and nutrient solutions must be adjusted to accommodate for that factor. It is also susceptible to pH shifts which creates the need to continuously monitor the ph Levels of your system.
Although rockwool has a high water retention capacity it has a restricted root environment and a low buffering capacity for water and nutrients. The water flow to plant roots may be hindered, even when the water content is apparently high.