Plants are green for camouflage.
It makes sense, after all, to be the same colour as your surroundings, and plants are mostly surrounded by the green colours of other plants. If a patch of grass in a field of cows was orange, it would stand out and the cows would see it and eat it. Plants are much better at camouflage than animals. If a bush grows to look like all the bushes around it, you will be completely unable to tell them apart.
Apart from that - yes, plants are just for decoration. Of course, this doesn't explain the existence of weeds. This had scientists foxed for a while, until someone pointed out that art is decorative too, and some of it is extremely ugly and pointless.
Plants are made up of three main parts - the Pretty Bit, the Edible Bit and the Greeny Bits. The pretty bit is of course the main part of the plant and is usually called the Petal-y Thing. The edible bit is known as the Vegetable. Sometimes it is poisonous, although you can still eat it if you really want to... The greeny bits such as the leaves serve no useful function. At least, we can't find a reason for them, so they don't have one, okay?