"Behold" is one of my favourite words. It's a marker of something that someone really wants you to pay attention to. Look! Pay attention! Behold!
I love the word behold. It's an archaic word, one that not many people use anymore. It's sounds quaint, old-fashioned. But it's a great word. Most people will say "look" nowadays, and it's really the same word. It's the Greek word ἰδοὺ, which really means "look!" or "pay attention!". Behold!
Twice near the end of Matthew 3, after Jesus is baptized, Matthew uses the word to draw our attention. First he draws it to the heavens being opened when Jesus comes up out of the water (Matthew 3:16), and then he draws our attention to the word of the Father: "This is my beloved Son, when whom I am well-pleased" (Matthew 3:17).
And then Matthew reserves the word until after a remarkable scene between the devil and Jesus in Matthew 4, when Jesus is tempted to sin and overcomes the temptation by the power of the Word of God. There is really much to behold in this passage, but we don't see the word again until Matthew 3:11, when angels ministered to Jesus.
Quite remarkable, really, is the restraint Matthew shows. I would have been sorely tempted to write behold! every half-sentence or so. But it's only when the angels show up that Matthew includes it again. It's not that he wants us to consider that the main part of the story. But it does show us something about God's provision.
Hebrews 1:14 asks us rhetorically, "Are [angels] not all ministering spirits sent out to serve for the sake of those who are to inherit salvation?" The expected answer is, of course, yes they are. We will inherit salvation as brothers and sisters in Christ. The angels serve the firstborn of all creation, and they serve for our sake too.