I use Sleep as Android for:

1. Great softer sounds that can be adjusted to gently increase alarm audio. I use bird sounds.
2. It can control lighting too.
3. It's smartwake looks at my sleep stage to choose the best time to wake on the window of time I allow. It uses my watch to measure sleep stages, but works with numerous other watches, chest straps, etc. or also has its own Sonar function using ultrasonic to detect stages from movement.

But chiefly it also has other stuff I use apart from the waking alarms such as sleep stats, warning for bedtime, lullabies when going to sleep, anti-snoring measures, noise recordings, 30 min power nap management, etc.