It’s obvious that you don’t want to waste food. When eating with your own family or best friends, it’s of course ok to finish all the food. However, in a more formal setting you’ve got to leave some on the plates. Here’s why:
When having a meal, try to avoid eating everything off the plates in the middle of the table. By leaving some food it shows you are full. If you finish all the food it is considered impolite and means you are still hungry! The host will feel embarrassed and order more food (even if everyone is actually full).
Finishing the food on your plate is however fine, as long as you don’t clean the plate where the dishes are being serve on. Leave a bit on the plate to show some modesty.
Also on a day like Chinese New Year leaving a bit behind means you have an extra spiritual food reserve through out the year.