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Scenario 1: Surfing the Internet
A typical threat scenario while surfing the WWW might be as follows:
The user works with his preferred browser and visits a page with a link to a script. The browser downloads the script and executes it. The script collects available information and sends them to a foreign server via the Internet.
In the worst case, a browser-bug can grant an attacker full access to all the data of the user because the browser runs with the privileges of the user. Indeed, UNIX permissions prevent access to system-data and to data of other users, but access to the user-data is not limited.
In this case, it is useful to further restrict filesystem access for the browser.
Scenario 2: Working with an Office suite
There are similar security risks when editing Office documents which contain active content like macros. These macros are executed, when the document is opened. The user must rely on the Office suite, that the granted permissions are sufficiently limited.
"BadBunny" [1] is a proof of concept macro-virus for OpenOffice. It demonstrates weak points in OpenOffice and provides a basis for real-world malware.
This example also shows, that further restrictions on filesystem access are useful.
Scenario 3: Distributed DOS attacks
Software for distributed DOS attacks is often distributed by exploiting security vulnerabilities of installed software, such as a web browser. Often, the next step is to open an IRC connection to a foreign server and wait for further commands received via this channel.
However, for most applications there is no need to allow IRC connections. Thus, most applications should not have unrestricted access to IRC.