As Jen mentioned:
It's(ADA AS) clay, so it stays when you pack in slightly, also, using rocks/wood etc shores the sediment up.
You can do this with other sediments, but you have to keep tossing back upo where you want it.
also, you can grow in many species of plants such as HC without any water other than enough to soak the sediments well for 2-8 weeks first then after it's well grown in, add water and fish.
This is the Easiest method to start up a tank without dosing, water changes, any algae, completely cycled tank etc.
The first 2 months is the toughest and this avoids it all.
Just cover the tank, add light and sediment(ADA AS works great BTW and cost about the same most sediments, you can mist the plants if you use flourite black etc with a NPK and trace solution 2x a week otherwise)