I am sure my child sees clearly, so why do I need to have their eyes checked?” 

Just as a child grows very quickly, the eyes can also change. Once a child is in school, they should be having annual examinations, and more frequently if their eyes are changing or if they are having difficulties in school with learning. Children are often unaware they have a vision issue as they believe what they are seeing is normal.
Did you know:

• 80% of learning is visual for a child.
• 1 in 4 school-aged children have visual problems
• Children with visual problems are often misdiagnosed as having a learning or behavioral disability

The earlier an eye health or visual problem is identified, the more likely it can be corrected. Conditions such as amblyopia (lazy eye) and strabismus (crossed or wandering eyes) are best treated when a child is young, as a child may see clearly out of one eye, but not out of the other.

If your child is not seeing properly and comfortably, it can impact on their ability to learn.

Source: aao pamphlet