I've used HasOne relationships for 1:1 relationships, but those are rare. I haven't considered using them to scope down relationships, like having one default payment method in a set of n methods.

<?php
 
class User extends Model
{
    public function paymentMethods(): HasMany
    {
        return $this->hasMany(PaymentMethod::class);
    }
 
    public function defaultPaymentMethod(): ?HasOne
    {
        return $this->hasOne(PaymentMethod::class)
            ->whereDefault();
    }
}
 
$user->defaultPaymentMethod;

After reading Tim's post, I have a feeling there are some places where I needed this but didn't think of it at the time…