Why Does Shakespeare Use the Theme of 'Appearance and Reality'?

Why does Shakespeare use the theme of ‘Appearance and Reality’?
The Merchant of Venice, a play by Shakespeare, is a love story between Portia and Bassano in which Bassano borrows money from Antonio who in turn borrows from Shylock. One of the main themes of this play is appearance and reality and this can be seen in Act 1 and Act 2. Bossanio, shylock, Portia, Jessica and Antonio are the main characters in the play. The relationships between them are not whet they seem at first
An example of mismatch between appearance and reality can be seen when Bossanio chose the lead casket instead of the golden or the silver casket. Which shows that he cares more about the content than the appearance of the casket. Portia says, “All the glisters is not gold” on page 69. This quote confirms that the main message that Shakespeare wants the readers to learn is everything is not what it seems like.
Another example can be seen between Shylock and Antonio. Antonio goes to Shylock to borrow three thousand ducats for three months, and he signs a flesh bond. “I would be friends with you…” p.29. This shows that he is pretending to be nice and kind to Antonio, when he’s actually plotting to make Antonio sign a contract, that Antonio wouldn’t be able to honor, as he is not capable to pay him back on time, and he will have to die.
NO CONCLUSION
What Is Shakespeare trying to teach us by presenting the theme of Appearance and Reality?