“Is it something to do with breathing through my diaphragm?
Does it make meditating easier, better or more optimal?
Or is it something else unrelated to meditation?”
It doesn’t.
Your back doesn’t need to be straight when meditating.
Meditate in a position that you find comfortable.
One that allows you to fully relax your body.
The seated meditation position that is most often associated with meditation isn’t necessary.
Meditate lying down on your bed or sat in a reclined chair with head support.
Whilst you don’t need a straight back to meditate, a straight back is better for your health.
Edwin Leo 