All manned spacecraft have had seats in an "on-back" angle for launch. What is the benefit of this seating position for a manned spacecraft launch?
The human body tolerates G-forces best if they're perpendicular to the spine.
When you apply a G-force along the spine, the blood will gravitate towards the feet, leaving the head starved of blood, which induces loss of consciousness. Having the head at the lowest position helps to prevent these blackouts.
3G is tolerable even if you're facing down, by the way. The seating position in capsules is chosen to support the body during landing (the forces in reentry and landing are larger than during launch).
Interestingly, the Soyuz capsule has seats that change orientation at landing time.
They are mounted appropriately for absorbing G-Loads the best at launch. Then seconds before landing, the seats change orientation to prepare for better shock absorption during landing.
Apollo's seats had shock absorbers for landing, and required certain depths for 'travel' which was an issue they had to consider when they designed the Skylab rescue Apollo for the worst case of a disabled Apollo at Skylab. They would need to carry at least 4 people, (one pilot up, 3 down, more likely 2 up so they could work as a team and three down) which meant 2 rows of seats, which the travel requirement caused issues with making it all fit.
let us take the shuttle
"Space Shuttle (takeoff): 29 m/s2"
which means 29/9.8 $\approx$ 3g In such a case the force acting on the body would be tremendous, so if your seating position is in the opposite direction, the force is strong enough to crush your ribs (chest) and the broken bones from the ribs can even penetrate leading to death or it can cause breathing problem and even cardiac attack.
If the seating is "on-back" angle for launch much of the force during the acceleration would be transferred to your back side of the body to the chair so less force is felt by your heart.