A big advantage of the robos is that they time the rebalancing a little better. Manually rebalancing your own Vanguard ETFs at the same time once a quarter is pretty arbitrary - it's based on what's convenient to you, but it's not necessarily what's mathematically best for your portfolio.

If your allocation percentages remain very stable, you might not need to rebalance at all at the end of a quarter. If your allocations fall way out of whack, you might want to rebalance a portion of your portfolio earlier, and robos handle that timing for you.

I'd be surprised if the better timing doesn't provide 0.25% of value, not to mention it's just one less thing to have to think about.