Why is the dome of the rock important to christianity?

Actually, the Dome of the Rock's importance to Christianity is indirect. It's more about its location than the structure itself.

The shrine stands on the Temple Mount. This site is profoundly significant to Christians. It's where the Second Jewish Temple stood, a place central to Jesus's life and ministry. The Gospels mention Jesus teaching in the Temple courts. He also famously prophesied its destruction. So, while the Dome is a revered Islamic shrine, for Christians it serves as a powerful landmark on ground that is sacred to their own history and scripture.