A Plastic Egg is frequently used for the Egg Bag routine. It is certainly better than a wooden one, being very lightweight, whereas a wooden egg will cause the bag to sag and give away its presence. 

The Plastic Egg is also perfect for productions where a large quantity of something is desired. Since the egg is itself a rather comic thing, the production of a large number of them is sure to raise a good crop of laughs. 
It is also possible to make a Stodare Egg out of a Plastic Egg for the Silk to Egg trick. It can be done by tracing a penny on one side, then with an exacto knife cut out a hole for the reception of the silk. Sand any roughness away and it's ready.

A Plastic Egg is just that, made of plastic and hollow.