What are custom pivot models for?

Question

Laravel

Wanted to use it to "Eager Load" additional Many-to-Many relationships, but it doesn't work.

Why are they needed in practice?

class Product extends Model
{
    public function features(): BelongsToMany
    {
        return $this->belongsToMany(Feature::class, 'feature_product_pivot')
            ->using(ProductFeature::class)
            ->withPivot(['value_id', 'unit_id']);
    }
}
class ProductFeature extends Pivot
{
    protected $table = 'feature_product_pivot';
    protected $with = ['value', 'units'];

    public function value()
    {
        return $this->belongsTo(FeatureValue::class, 'value_id', 'id');
    }

    public function unit()
    {
        return $this->belongsTo(FeatureUnit::class, 'unit_id', 'id');
    }
}