Why An electronic keyer is a CW sending aid: paired with a paddle, it generates strings of perfectly timed dots on one lever and dashes on the other, with the correct 1:3 element ratio and the proper spacing between them. A speed control sets the dot length, so the whole character timing scales together and your fist stays machine-steady. Most modern transceivers have a keyer built in, along with memories that can play canned calls or contest exchanges.
Watch out Automatic transmit/receive switching is the job of VOX or break-in (QSK) circuitry, and the delay for antenna changeover is part of that same T/R sequencing, not the keyer. The computer-to-radio interface for PSK31 or RTTY is a sound card interface or a digital modem, not a keyer.
Keyer = key helper: squeeze the paddle, it makes the dits and dahs. Left/right, dot/dash, perfect timing.