When is it necessary to use the Shift key during startup for an Intel-based Mac?

Using the Shift key during startup for an Intel-based Mac is necessary to start the system in Safe Mode. Safe Mode is a diagnostic mode that restricts what runs on your Mac during startup, allowing you to troubleshoot issues such as persistent problems caused by software, extensions, or incompatible applications. When you initiate Safe Mode, the Mac performs some checks and prevents certain software from loading, which can help you identify and resolve issues more effectively.

In contrast, entering Recovery Mode typically involves using a different key combination, while booting from an external disk would require holding down the Option key during startup. Accessing BIOS settings is not applicable to Intel-based Macs as they utilize EFI (Extensible Firmware Interface) instead of a traditional BIOS. Each of these options serves a specific purpose, but Safe Mode is uniquely initiated with the Shift key, which is why it is the correct choice in this context.