A show presented as tonight's conversation is usually recorded in the late afternoon. The lead time is required by the production chain that sits between the taping and the broadcast.
Post-production needs a window
A taped hour runs long and must be cut to a broadcast length that accounts for commercial breaks, so editors remove material after the taping ends.
That editing also fixes technical problems, tightens a monologue that ran slow, and drops segments that did not work in the room.
Doing this properly takes hours, and a live broadcast would remove the option entirely, leaving whatever happened in the studio as the finished program.
Standards review happens before air
Broadcast television operates under content rules, and networks maintain standards departments that review material before it goes out.
An afternoon taping gives that review time to flag language or content and for the production to cut or bleep it rather than issuing an apology afterward.
Legal review works the same way, particularly for comedy about identifiable people and organizations, where a claim is far cheaper to prevent than to defend.
Guests are scheduling around everything else
Guests are usually promoting something and appear on several programs within a short window, often in the same city on the same day.
Afternoon tapings fit that circuit, allowing a publicist to route a client through multiple appearances without asking anyone to work at midnight.
Performers on tour, actors in production and athletes in season have obligations that make a late evening commitment impractical, so the tape time follows their availability.
Studio audiences will not come at midnight
These shows depend on an audience for reaction, and comedy without laughter reads very differently on camera.
Filling several hundred seats is far easier in the late afternoon than after midnight, especially in cities where getting home late is a real obstacle.
Warm-up comedians, seating and audience management add their own time before the taping starts, which pushes the schedule earlier still.
What live coverage costs and buys
Some programs go live for specific events, accepting the loss of editing in exchange for genuine immediacy and the reaction it generates.
The tradeoff is real: live broadcasts carry higher risk, need larger standing crews and cannot be adjusted once something goes wrong.
Taping in the afternoon is the compromise, buying a same-day feel with the safety and cost structure of a recorded program.