After 3000 hrs in game I think maybe I have made certain in game observations.
They work as an added deterrent as does thickness and layering. ERA adds even more defense and turns away large amounts of damage in the right setup. I built this as part of a large selection of ground forces. The 500mm cannon on a small inexpensive tank was the goal. Ball in net with this one.

nope they actualy effect shots, hard to tell but a 4m has a high chance to bounce if hit straight on like the 1m but they are affected differently on which side they are hit on, im sure they used to be actual diagonal shapes ages ago, the player to block hit boxes were changed and the balistics have slightly too (collisions), like explosives, they seem weak but they pass through armour and cause dammage to the insides of structures with their "spalling" effect, thats why spaced armour is usualy used and so is internal ERA armour

I'm not talking about the angle of a actual block for armor I'm talking about the 1m through 4m slopes, the hit boxes are still blocks even if the seeable part is a slope.

Also Sloping Effects Drag In AND Out Of The Water.

well it seams to work for me.....

Sloping doesn't help armor by the way, its just for looks.