Tails attempts to provide an operating system that appears uniform for all users, boots from a USB flash memory and leaves no trace on the host device. It also routes all traffic through Tor (except for captive portals) and provides Tor Browser.
My naive view about a standard operating system in comparison to Tails:
- Tor Browser is potentially fingerprintable because of the OS characteristics (fonts, installed apps, etc.). Tor Browser does a lot to resist fingerprinting, but had vulnerabilities in the past and should not be assumed to be invulnerable.
- The user’s uniquely-identifying data, sensitive information and operating system logs that persist (whether or not by user’s permission) on their device are compromised if any browser or other app is hacked.
- A lot of work must be done to harden a fresh install of a standard Linux operating system against security/privacy risks: restrict root access (the default might be sudo everything works!), disable logging, restrict the firewall (the default might be accept all traffic!), MAC address randomization, disable IPv6 or prevent IPv6 MAC address leakage, disable automatic connecting to WiFi, secure APT, install Tor and related software, etc. Tails does some of these things out of the box.
Aside to your question, what reasons are there not to use Tails or Whonix? If Tails or Whonix satisfies your use case, whether you have a “high risk threat model” or not, just use it.