The icons are meant to be more convenient for you when your looking for particular meldable items so if your looking to see if you can meld a certain materia onto pants you always can look at the pants icon which is always in the same place and just scroll down the list and see if it is lit up or not. In the text based system the text would move around so you would have to potentially reread the entire list to see if what you wanted was in it with the icons you should never have to do that (This is assuming you can see/understand the icons).