In English usage the word behold is a bit archaic, or old fashioned.  But it is a useful word in that it expresses an idea that is bigger than it might first appear.  Behold means to look at something, but beyond that it means to examine that something in order to understand it.  The word comes from Germanic roots and was originally bihaldan, meaning to thoroughly hold, or grasp.  Do you grasp my meaning?  The key word in that definition is “thoroughly”.

To behold is more than to just look at.  It is to carefully examine, to understand at depth.  To behold the man is more than just to look at him.  It is to examine him, figure out how he works, to grasp what he is, where he came from, and what he means  The example in the question is illustrative.  “Behold the Lamb of God” is inviting the observer to consider who the Lamb of God is, what does it mean for this to be a lamb and not only that but the lamb of God.  When we behold the Lamb of God, we call to mind the sacrificial nature of a Lamb and we learn to appreciate that God provided his own sacrificial lamb to take away the sins of the world. The use of the word behold invites us to dive deeper into the meaning of things and to hold the thing beheld in higher esteem and as supremely precious.  The word invites us to a deeper appreciation because we are looking at something that requires more than just a casual glance.