I was spoiled. Not spoiled with material indulgences; but with attention and care. My parents - and my paternal grandparents as they only have one grandchild - need not divert their time to care for other children. Mother had the time to read bedtime stories and build sandcastles with me. In hindsight, I had a golden childhood.
Not having to feed a football team has its merits financially. I was able to go for piano lessons, art classes and tuition. My friends with multiple siblings are not as fortunate; I once had one lamenting about not being able to go for music lessons because there simply wasn't enough money. I was also able to travel to other countries since I was young. Basically all my needs are fulfilled, and I am ashamed to name the unsensible wants that I’ve requested for and gotten.
Imagination was my best companion; it still is. I have no problem entertaining myself for hours on end. As a child I play pretend, read, dream all by myself. My parents always caught me talking to friends; I still do - shhhh don’t tell my parents. I used to have an invisible dog; I wonder if my Mongrel can see him.
With all these solitude I learnt to be independent from a young age. I had no one to cling on to when I started the first day of school; no older sibling to hide behind when I made a mess. Maybe it has nothing to do with actual independence but more like being forced to do everything alone just because there's no way your father will play doll with you. This worked out for me just fine.
I don't think there’s much obvious benefits. Being bathed in love and attention is the major phenemona I see in only children like myself. However I have only lived two decades and perhaps have not experienced more pronounced benefits stemming from only childness.