Save you money. If you don't need anything fancy and just want a basket of ETFs and index funds, a robo-advisor can handle this with ease. No need to pay more for these very basic offerings; instead, that extra money can go toward your retirement plans.
Respond 24/7. Unlike humans who need to sleep, robo-advisors are computer programs that can be reached at any time of the day or night. If you want to check your account balance or see how the robo-advisor has reallocated your account funds, it's simple to check when you have the time to sit down in a quiet place and focus.
Execute trades efficiently. Although you are highly limited in the types of accounts you can invest in, trades are executed in an efficient manner -- often, the moment your accounts go outside of their rebalancing bands. With a human, you'd have to wait until they are able to execute the trade manually, which could result in more loss.
Tax loss harvesting. It doesn't seem like a big deal, but tax-loss harvesting can save you so much money in taxes that it should be considered one of the most important jobs of a robo-advisor. With tax-loss harvesting, you essentially sell specific investments at a loss to offset gains in other investments, which is a very time-consuming job for a human.