Because it shields the fact you're going through Tor. Tor network traffic alone is a red flag that you're doing something shady, even if it isn't sufficient proof.
A comical example is when a Stanford student used Tor to call in a bomb threat to get out of an exam. Turns out he was one of two people on the entire campus network using Tor. The police talked to him and he admitted to it.
(Side note: if you're going to do crimes don't admit to them when asked.)
> The most common rationale behind this setup is to hide Tor usage from an ISP or circumvent censorship of the Tor network. This is unnecessary as you can hide Tor usage and circumvent censorship by using bridges. You can either use the bridges that are included in Tor Browser for this, or request other bridges from in any of the ways described here. A bonus of bridges is that they don’t leave a money trail, which VPNs often do. [...] Even if you were to end up on a watch list, it would be a uselessly large list as Tor has more then 2 million daily users. It strikes me as very naive to imagine that someone powerful enough to trace you over the Tor network will be stopped by a $5 a month VPN service.
A comical example is when a Stanford student used Tor to call in a bomb threat to get out of an exam. Turns out he was one of two people on the entire campus network using Tor. The police talked to him and he admitted to it.
(Side note: if you're going to do crimes don't admit to them when asked.)