The Renaissance was a meshing of religion and art in a way that is difficult for modern viewers to understand. In order to fully understand the Renaissance, we need to put ourselves in context and try to understand the different forces at play in the time period of the art. If most art was meant to be an altarpiece, then taking it away from the church removes a huge chunk of our ability to see the art in context. So we need to pay attention to the original purpose, not just where the art is now.