VoiceOver increases accessibility for blind and low-vision users, and for users with certain learning challenges.
To make sure VoiceOver users can use your app, you might need to supply some descriptive information about the views and controls in your user interface. Supporting VoiceOver does not require you to change the visual design of your UI in any way.
When you use standard UI elements in a completely standard way, you have little (if any) additional work to do. The more custom your UI is, the more custom information you need to provide so that VoiceOver can accurately describe your app.
Making your iOS app accessible to VoiceOver users can increase your user base and help you enter new markets. Supporting VoiceOver can also help you address accessibility guidelines created by various governing bodies.