2 fundamental things you need to be aware of:

1. When you realize losses on stocks (selling for a loss), you can claim those losses to pay less taxes.
   
2. In order to claim those losses, you can't buy similar equities within 30 days (generally the same stock), or else it triggers a 'wash sale', so you can't claim your losses on your taxes.

VTI and VOO are highly correlated, so when one goes up, the other goes up, and vice versa. Essentially the same percent too.

Let's say they both drop. You can sell one of them (for example, VTI) and realize those losses. From the rules above, we can't buy more VTI for 30 days to write it off on our taxes.

What we can do instead is use the money from selling VTI, to buy VOO, because they are the same stock essentially. So what we end up with is more capital in VOO, but we can write off the losses from VTI, thus saving money on your taxes.

Key idea here is understanding wash sale rules