You'll be more comfortable, you won't be wasting muscular effort, and you won't fatigue so early in the day. That's the point. And if someone who is going to share the chair changes it, don't complain. Consider that you want them to be safe, just as they should care about you. When everyone knows how to use the chair, it's no problem when each sets it for their needs.
Some chairs are just plain wrong for a given person.
There is no way to sit against the back of the chair when this is the case unless you rotate the pelvis backward and round out the lower curve of the spine. In other words, you slump. Since the spine isn't able to do its strtuctural work anymore, that means that muscles have to do more work. That includes the neck and shoulders, which have to pull the head back to compensate for the slumped posture - especially if the monitor is too high. Now we have conditions that increase the chance of thoracic outlet problems, possibly impeding blood flow into the arms through the shoulders.