Some operating systems automatically add or preserve file extensions when renaming files as a usability feature to prevent users from accidentally breaking file associations. File extensions (like .docx or .jpg) tell the system which program should open the file. Hiding extensions by default, but temporarily revealing or enforcing them during rename operations, reduces the chance of a user mistakenly deleting the crucial extension part while changing the file name, which would render the file unopenable by its intended application.

For instance, Windows File Explorer typically hides known extensions by default. If you rename "report.docx" to "report_final", the system will automatically retain ".docx" and save the file as "report_final.docx". Similarly, macOS Finder obscures extensions for certain file types but will add or preserve them during a rename action to ensure application linkage remains intact. This behavior is prevalent in general-purpose desktop operating systems prioritizing user-friendliness.

The primary advantage is protecting non-technical users from rendering files unusable by accidentally deleting the extension. However, hiding extensions can mask security risks; malicious files might disguise themselves (e.g., "document.txt.exe" appearing as "document.txt"). Future OS developments might offer smarter default warnings for potentially dangerous extensions while maintaining ease-of-use. This balances system integrity with usability.