I'm not sure how many times one wakes up per hour before the apnea is considered severe. My sleep apnea must be at least moderate. Uncontrolled, I imagine I wake up 20-30 times per hour.
When you wake up multiple times per hour, due to apnea, a couple of things result. One, you never reach deep sleep. You won't know this. You think you've slept 8 hours, or whatever. But waking up many times per hour, for mere fractions of a second, prevents you from reaching the deep restorative sleep the body needs. It also jolts the heart when one wakes up repeatedly. That's not good. Yes, controlling that helps.
We all need to reach deep sleep each night. On nights when my apnea might not be controlled as much as on other nights, I can usually tell the next day. There is a level of fatigue telling me I did not sleep as well as I would like. It's the cumulative affect of minimizing restorative deep sleep night after night that lays waste to daytime consciousness and energy. It affects the mind and the body.
I can only speak for myself. I know people who simply live with the apnea. Maybe if one has it to a minor degree, you can do that. But, for myself, allowing my body to reach deep sleep, and allow that deep sleep to be uninterrupted, which is what the CPAP machine allows, helps my daytime consciousness and energy.
Bottom line: sleep apnea interrupts deep sleep. Our bodies, and our minds, need uninterrupted deep sleep. It's that simple.