Some features that Betterment offers for example

* automatic rebalancing

* tax loss harvesting

* tax co-ordinated investing - looks at both your taxable and tax exempt/deffered accounts and directs funds appropriately (for example, puts more tax inefficient assets in your tax exempt accounts)

You can of course do this on your own as well, so it's up to you to decide whether the additional fee is worth it or not. Also, not all robo advisors offer the same features - but most offer automatic rebalancing at a minimum.