Question: Why is the `Visitor` trait so important?

[cactusdualcore]

Custom deserialization code seems to turn very complex quickly, mostly because of all the visitors. I wonder, what was the original reason this API was chosen? Is there a reason the crate decided not to expose "simpler" APIs, like in the example below?

enum Value {
    Name(String),
    Age(u32),
}

struct Student(HashMap<bool, Value>);

impl<'de> Deserialize<'de> for Student {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>
    {
        #[derive(DeserializeOrEnumerate)]
        enum Field { Name, Age };
     
        let name = None;
        let age = None;

        while let Some(field) = deserializer.deserialize_field<Field>()? {
            match field {
                // filling those `Option<T>` vars...
            }
        }

        Self { /* ... */ }
    }
}