A Brief History for Wax Seal

In the middle ages, seal was a symbol of authority and power and used by the aristocracy and the Church for use in issuing official decrees and authenticating documents. As the practice spreads, each individuals had their own seal, as many were illiterate, the seal is used as a signature. 

In not so distance past, sending a letter can be quite expensive. The postage fee is based on the distance and the number of sheets. An envelope is counted as an extra sheet, hence doubling the fee. So letters were written in a single sheet and sealed it shut with wax to avoid the extra expense

Today, using wax seals is as unnecessary as handwritten correspondence, and yet like many old traditions, it is pleasurable to practice, and adds a bit of personal distinction and panache to your communication.

Quoted from Art of Manliness