Offer your thoughts or opinion on the input financial query or topic using your financial background.

Why might a robo-advisor service like Betterment be preferable to just buying a single well-performing index fund like SPY?
The reason diversification in general is a benefit is easily seen in your first graph.  While the purple line (Betterment 100% Stock) is always below the blue line (S&P), and the blue line is the superior return over the entire period, it's a bit different if you retired in 2009, isn't it?  In that case the orange line is superior: because its risk is much lower, so it didn't drop much during the major crash.  Lowering risk (and lowering return) is a benefit the closer you get to retirement as you won't see as big a cumulative return from the large percentage, but you could see a big temporary drop, and need your income to be relatively stable (if you're living off it or soon going to).   Now, you can certainly invest on your own in a diverse way, and if you're reasonably smart about it and have enough funds to avoid any fees, you can almost certainly do better than a managed solution - even a relatively lightly managed solution like Betterment.  They take .15% off the top, so if you just did exactly the same as them, you would end up .15% (per year) better off. However, not everyone is reasonably smart, and not everyone has much in the way of funds. Betterment's target audience are people who aren't terribly smart about investing and/or have very small amounts of funds to invest.  Plenty of people aren't able to work out how to do diversification on their own; while they probably mostly aren't asking questions on this site, they're a large percentage of the population.  It's also work to diversify your portfolio: you have to make minor changes every year at a minimum to ensure you have a nicely balanced portfolio.  This is why target retirement date portfolios are very popular; a bit higher cost (similar to Betterment, roughly) but no work required to diversify correctly and maintain that diversification.