There are quite a few reasons to place a sign (or 10!) in your garden.  If you would like to identify what plant is growing where, you can mark them with a sign.  Say you keep bees in an area or would like to advertise that your garden is pesticide free; a sign is a great way to make it publicly known.  Many community gardens use signs to distinguish individual plots, publicize work days or discourage vandals from stealing produce.  Maybe you just want to add some beautiful art to your garden and proclaim your name!  Whatever the reason, installing a sign is always a good idea.