"Show" is very general, whereas "showcase" is much more specific and implies that you are presenting something in a grand manner to emphasize its positive attributes. "Showcase" can only be used in certain contexts. For example, "The advertisement will showcase many new products and show consumers how to use them." You can also say "The advertisement will show many new products and show consumers how to use them." However, you CANNOT say "The advertisement will show many new products and showcase consumers how to use them." This last sentence is wrong, because you can't "showcase" someone how to do something. I hope that makes sense :)

Collin's dictionary says the following: "You use showcase to refer to a situation or setting in which something is displayed or presented to its best advantage." Show is simply show but in any situation not necessarily showcased.