Engagement is an attempt to gain social recognition for the seriousness of one’s relationship without actually having to make any commitments – therefore it is superfluous.
Engagement is essentially a promise to make a promise. If I promise you that on Wednesday I will promise to give you $5 on Friday it would be considered ridiculous. Why don’t I just promise to give you $5 on Friday in the first place? The analogy holds for marriage. If two people are going to be married in three weeks, and they use the idea of engagement to tell people of this, I can see some sense in it, but when two people have no idea when they are going to get married, and they go around saying they are engaged, what are they actually saying to each other and everyone else?