On the top of most teapot lids there is a hole a millimetre or two across, sometimes hidden in the hollow stem of the knob. It is commonly explained as a place for steam to escape. That explanation is half right and misses the more useful half, which you can check at the sink in about ten seconds: start pouring with the lid on, press a fingertip over the hole, and watch the stream thin, stutter and finally hesitate. Let go and it recovers. The hole is primarily an air inlet.
Water leaving the spout has to be replaced by something. As the level inside the pot drops, the space above it grows; with no way for air to enter, the pressure in that space falls below the pressure outside, and the atmosphere pushing back on the spout soon balances the head of water trying to get out. The flow becomes a series of glugs as air forces its way backward up the spout in bubbles. Anyone who has poured from a sealed container knows the rhythm.
That is why the hole sits in the lid rather than near the spout. It has to open into the air space above the liquid, and the highest, driest point of that space is the crown of the lid. Put the opening low down and it would be submerged for most of the pour, at which point it would simply become a second, leaking spout. The lid is also the part that lifts off for filling, so the opening stays where it can be checked and cleared.
A small experiment with three results
Pour with the hole open and the stream is smooth. Cover it and the stream weakens; if the lid fits loosely, water may appear at the seam between lid and rim, because that gap has become the easiest path for air. Fill the pot almost to the brim and the effect is strongest, since there is very little air inside to expand before the pressure drop stops the flow. Each of these outcomes says the same thing: what the pot needs is incoming air, not outgoing steam.
Steam does use the hole, and that matters most in the first minute after boiling water goes in. Hot vapour and expanding air need somewhere to go or the lid will lift and rattle. But the venting requirement is intermittent, while the inlet requirement applies to every single pour for the life of the pot.
What the spout contributes
The spout is not just a tube. Its upper edge usually sits level with or slightly above the highest intended water line, so the pot can be filled without dribbling out. A sharp lip and a slight return under the tip help the last of the water break cleanly instead of running down the outside. Those details are about ending the pour; the hole in the lid is about letting the pour happen at all.
On a wheel-thrown bowl the maker trims a foot because the kiln will not tolerate glaze there. On a teapot the maker drills a hole because physics will not tolerate a sealed lid. Both are small, unglamorous operations imposed by the process rather than chosen for appearance, and both are the first place to look if you want to know whether the object was made to be used.
It also explains a common complaint. Tea scale or limescale can close a pinhole over time, and the pot then pours in fits, or spits from under the lid. A fine needle and a rinse restore it. It is worth comparing a thick glaze at the lid seat here: a heavy glaze run can narrow the gap between lid and rim, and a crackled surface will show where the piece was handled most. Neither is a fault in the design; both change how freely air can move.
So the pinhole is not ornament, and it is not really a vent. It is the second opening of a vessel that needs two: one for water out, one for air in. Cover it and the pot politely refuses to pour.