Found this meaning in Wikipedia... Now I know why is there a need...
"An engagement is a promise to marriage, and also the period of time between proposal and marriage – which may be lengthy or trivial. During this period, a couple is said to be affianced, betrothed, engaged to be married, or simply engaged. Future brides and bridegrooms are often referred to as fiancées (brides) or fiancés (grooms). In the modern era, some women's wedding rings are made into two separate pieces. One part is given to her to wear as an engagement ring when she accepts the marriage proposal, and the other during the wedding ceremony. When worn together, the two rings look like one piece of jewelery."