It was not a total block-out, but was a rather thin material. A last vestige remains in the veiled cross of Good Friday. And even that is optional. The idea was, of course, sin. Sin darkens vision, the vision of God, of faith.
Penitence helps remove that.
Besides, it is to cultivate reverence and awe for holy things. The cult of the secret. Eastern churches often use an icon screen back of which the liturgical action takes place. In great cathedrals of the past, the high altar was not necessarily visible to all. The choir stalls in front of it, sometimes a rood screen or other piece blocked the view.