Why An electronic keyer is a circuit that works with a paddle to generate perfectly timed Morse dots and dashes: hold one lever and it sends a stream of dits, hold the other and it sends dahs, all at the speed you set. The operator still decides which characters to send, so it assists manual sending rather than replacing it with automatic text. Many modern transceivers have a keyer built in, with a speed control in words per minute and a jack for the paddle.
Watch out The voice-activated switching choice describes VOX, which keys the transmitter when you speak, and the transmit/receive antenna switching choice describes a T/R switch or relay, not a CW keyer.
Keyer = key + timing. Paddle in, evenly timed dits and dahs out.