Not everything in virtual worlds are indoors. Sometimes you have to go out in the sun.
Plants and trees are often low polygone stuff that can fill up between the buildings or complex objects. Its important to place some open spaces in your simulation. We are not always indoors in real life. Well maybe i am. Have to buy a laptop i can take out on the grass.
But anyway making open spaces also saves polygons and textures for the more confined spaces of a game or simulator and makes it more exiting when you go from inside to outside.
Another good thing is that there are a lot of people that create plants for free ready to download. So it also saves you some money.
Another very nice way to create a outdoor landscape is to use water. Water makes a moving and often sounding addition to the open space and puts in a little action if your plants are static.