The introduction to Mark written in the ESV had an interesting point: “In addition to Jesus, Mark features three main groups of people: the disciples, the crowds, and the religious leaders, none of whom understand Jesus.” It’s a very good point; Mark devotes a lot of space to Jesus telling his disciples, and especially the apostles, over and over again, that he was about to die and be raised up, and each time he told them, they simply wouldn’t or couldn’t understand what he was trying to say. And it’s not just at the end, either: already in Chapter 8 Jesus is telling the disciples that he’s going to die and rise again—but this is the famous passage where Peter, having none of it, rebukes Jesus for saying this. How could Jesus possibly die before saving his people? Of course Jesus rebukes Peter even more harshly, calling him Satan, and telling him that he has in mind the things of man rather than the things of God—which is a big part of the apostles not understanding Jesus’ teaching: they’re looking for a political messiah, but Jesus is a spiritual Messiah. (Yes, I spelled it “messiah” and “Messiah” on purpose.)
Mark doesn’t devote any space to Jesus’ early years, the book starts with Jesus already grown up, and, starting with his baptism by John, going right into his ministry. Mark’s focus doesn’t seem to be on the life of Jesus, rather it’s on his teachings: he is a man, but he’s also God (and our king), and he has to suffer and be sacrificed in order for us to have a proper relationship with God. Once we do, he calls us to be humble, serve others, and follow God.
Another reason that new Christians are sometimes pointed to the book of Mark is that Mark is writing to a wider audience, not just to Jewish Christians, so he takes care to explain Jewish customs when they come up in the text. You can probably read Mark pretty easily, even if you don’t know a lot of the Old Testament, or have a deep understanding of Paul’s epistles. But even if you do know the other parts of the Bible, it’s worth coming back to Mark from time to time, and getting a simple, precise reminder of who Jesus is, and why we should care. It’s only 16 chapters, so it can easily be read in one sitting.