Data in transit is information that is actively moving from one point to another across a network, for example an email being sent, a file being uploaded to a server, or a message travelling between two applications.

While this data crosses switches, routers and links, it passes through equipment and paths that the sender does not control, so an attacker who taps the line can capture the raw packets.

Encryption transforms the readable plaintext into ciphertext using a cryptographic key, so anyone who intercepts the traffic sees only a scrambled, meaningless stream instead of the real content.

The primary benefit is therefore confidentiality during transmission: even if the packets are sniffed or captured, they cannot be understood without the decryption key.

Physical theft of a device is countered by encrypting data at rest (full-disk or file encryption), not by encryption in transit, so that option describes a different control.

Encryption adds processing overhead and so does not improve transfer speed, and stopping unauthorised users from joining the network is the job of authentication and access control, not of in-transit encryption.

Hence the primary benefit is that It ensures that data is unreadable to attackers during transmission.