This is what a fairly typical plate looks like after my daughter is finished. She's eaten about half of what was on the plate, 4 or 5 pieces of broccoli, the remnants of a puddle of ketchup. A small dish that had some vanilla ice-cream.
As someone who grew up in the clean-plate club, allowing my child to leave food was something that took some getting used to. It seemed wasteful. She had asked for seconds on the broccoli, and didn't finish it. I can hear my parent's voices, "You asked for more broccoli, now you have to finish it." For those who did not always have food in abundance, this is an understandable reaction. I try to serve smaller portions and let her ask for seconds and thirds. As she gets better at it, she will be serving herself family style.
Letting your child leave some food behind honors her inner voice– that voice that tells her she's full. Making a child clean her plate, or eat when she is not hungry will make it harder for her to tune in to the cues her body is sending. Losing touch with the feelings of fullness and satiety can later lead to a pattern of overeating. (One study showed that 85% of parents of kindergartners encouraged their children to eat more than they wanted to. "Just take two more bites of cheese, OK Sweetie?")
In addition, letting a child leave food on her plate means she will be more likely to try new foods. If she knows she doesn't have to finish something unfamiliar, she will be more likely to try it. Waste a little more now, but less later as she gets older and learns to enjoy a greater variety of foods.