What is a ridged cast iron griddle actually for, and why does everything I cook on mine stick and steam instead of searing?

For question 3 — it is genuinely better at a real category, and it is not steak.

Anything that gives off a lot of water and would otherwise stew: courgettes, aubergine, mushrooms, peppers, halloumi, spring onions. On a flat pan these release liquid, sit in it and go soggy. On ridges the liquid drops away and they actually char.

It is also the right pan for anything marinated. A sugary or oily marinade in a flat pan burns into a layer you then have to scrub; in a ridged pan most of it drains into the channels and only what is on the contact line caramelises.

Keep it for vegetables and halloumi and use the skillet for meat. That is not a compromise, that is the division the two pans were designed for.

The ridges exist to hold food up out of its own liquid, and understanding that answers all three of your questions.

On a flat skillet, fat and juices sit under the food and the food effectively fries in them. That is usually what you want. On a ridged pan the food touches metal only on the ridge lines and the liquid drains into the channels below, away from the contact. It is imitating a grill grate: the point is what runs off.

Which is exactly why your steak steams. Only about a third of the surface is touching hot metal, so two thirds of it is sitting over a trench of its own evaporating juice, and it cooks in that steam. The pan is doing what it was designed to do and you were expecting skillet behaviour.

So no, the marks are not the point — they are a side effect of the drainage. But if drainage is not what you want, you are using the wrong pan and no technique will fix it.

There is a technique fix for the sticking specifically, and it is heat, but not in the direction people assume.

Ridges have far less metal in contact with the food than a flat base, so they dump their heat into the first thing that touches them and recover slowly. If you preheat to the same temperature you use for the skillet, the ridge tops crash the moment the food lands, and food that touches metal below searing temperature bonds to it. That is your sticking.

So: preheat longer and hotter than the skillet, and give it real time — ten minutes on a medium flame, not three. Cast iron ridges are thick and heat unevenly across the pan. Then oil the food, not the pan, because oil poured into a ridged pan runs straight into the channels where it is no use and smokes.

And do not move it. If it is stuck, it is not ready; a properly seared ridge line releases on its own.

Cleaning, since it is the reason most of these end up at the back of a cupboard: the channels are the problem, not the ridges.

A chainmail scrubber is close to useless in there because it cannot reach the bottom of a narrow channel. What works is a stiff-bristled brush with the pan still hot, and hot water, straight after cooking. Once the fat in the channels cools and sets you have a much worse job.

If it has already built up a sticky varnish down there — which it may well have if it was inherited — that is polymerised fat rather than seasoning, and it is worth stripping and reseasoning once so you start from a clean pan.