Testing your GH is important in a planted aquarium. Most aquatic plants do not like hard water and if your GH is high, it means you need to take steps to reduce it.
Nearly all plants from a soft water area where the water is acidic and to get the best from your plants you need to replicate these conditions. Therefore regular testing of your GH means you have good understanding of how hard your water is.
Typically anthing from 4-8 GH is recommended for plants and discus tanks.