wight net mesh ?

I see lot of picks of grows with wight net mesh on top of the pants what are the pros of doing this ? at what height do you put the net ? will it help the plants spread out more

That mesh is to support the phatty buds from falling over. in a hydro set up the roots arent able to anchor the plant and the chances of it falling over increase..
its not always needed..depends on things like veg time and vareity

its also used in a method of growing called SCROG "screen of green" in this method the plants are trained to fill the entire screen so you fill all possible growing space and the light is distributed to the plants evenly..im new to the site but im sure you can find plenty of info on the SCROG growing method