It's an artifact of the technology they use. US launchers are hot launch: the motor ignites in the cell, and then leaps out under its own power. This has some disadvantages with having hot exhaust inside the ship.

Soviet launchers used a cold launcher. They pushed the missile out first, then ignited the motor after it was out of the cell. This is good for reducing exhaust impingement. But you can end up with a live missile not actually going up if the motor igniter fails. Angling the launcher ensures your live missile lands in the water, instead of on deck.

I've seen video of this, but having no luck searching for it.