Utilize your financial knowledge, give your answer or opinion to the input question or subject . Answer format is not limited.

Why might a robo-advisor service like Betterment be preferable to just buying a single well-performing index fund like SPY?
What is the advantage of something like Betterment -- which diversifies my investments for me but also charges a fee -- if I can just buy SPY on Robinhood for no fees and do better? Because Betterment is more diversified than the S&P, glaringly when it comes to non-US investments.  The US's economy is huge.  It represents 22% of nominal global GDP and 17% of global GDP (PPP).  While I think that the US's stability is good reason to be overweight US, being 100% invested in 22% of the market isn't well diversified.