If it were possible to use the same USB stick with another operating system, for example to store files to use on Windows, a virus in the other operating system could corrupt your Tails.
Many websites today require JavaScript to work correctly. As a consequence JavaScript is enabled by default in Tails to avoid confusing many users. But *Tor Browser* takes care of [[blocking dangerous JavaScript functionalities|doc/anonymous_internet/Tor_Browser#javascript]].
Tor Browser also includes a [[security level|doc/anonymous_internet/Tor_browser#security-level]] and the [[NoScript|doc/anonymous_internet/Tor_browser#noscript]] extension to optionally disable more JavaScript. This might improve security in some cases. However, if you disable JavaScript, then the [[fingerprint|doc/anonymous_internet/Tor_Browser#fingerprint]] of your *Tor Browser* differs from most users. This reduces your anonymity.