USB thumb drives (also called flash drives) serve several practical purposes in networking:
File Transfer: They are excellent for transferring Cisco router or switch configuration files between the device and a PC. This allows you to back up configurations, easily restore configurations, or work with them offline.
IOS Image Storage: USB thumb drives are convenient for storing and transferring Cisco IOS images, especially when you need to upgrade or recover network devices.
Why other options are incorrect:
to save the system log: While you might temporarily store log files on a USB drive, routers and switches generally utilize internal memory or a remote syslog server for logging purposes.
to save router hardware information: Hardware inventory information obtained from commands like show inventory is often copied and pasted to text files, which can then be stored on a USB drive. However, the drive itself doesn't directly save or store this hardware information.
to increase system memory: USB drives are not designed to act as additional system memory (RAM) for a router or switch.